feat: idle cash alert and category hierarchy (subcategories)

Two of the four ideas picked from the Maybe/Sure feature audit:

- Idle cash: a 5th proactive alert flags a depository account (checking/
  savings/cash) sitting at or above a configurable balance with no
  transaction touching it for a configurable number of days.
- Category hierarchy: categories can now have one level of subcategories
  (Groceries -> Alcohol). Dashboard/report aggregations roll a
  subcategory's spending into its parent's slice.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/alembic/versions/3b7c941bbadf_add_idle_cash_thresholds.py

@@ -0,0 +1,36 @@
+"""add idle cash thresholds to app_settings
+
+Revision ID: 3b7c941bbadf
+Revises: a71d9f0c3e58
+Create Date: 2026-08-25 10:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '3b7c941bbadf'
+down_revision: Union[str, None] = 'a71d9f0c3e58'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        'app_settings',
+        sa.Column('idle_cash_threshold_amount', sa.Numeric(14, 2), nullable=False, server_default='1000'),
+    )
+    op.add_column(
+        'app_settings',
+        sa.Column('idle_cash_threshold_days', sa.Integer(), nullable=False, server_default='60'),
+    )
+    op.alter_column('app_settings', 'idle_cash_threshold_amount', server_default=None)
+    op.alter_column('app_settings', 'idle_cash_threshold_days', server_default=None)
+
+
+def downgrade() -> None:
+    op.drop_column('app_settings', 'idle_cash_threshold_days')
+    op.drop_column('app_settings', 'idle_cash_threshold_amount')

backend/alembic/versions/8e2f5a13c9d4_add_category_parent_id.py

@@ -0,0 +1,32 @@
+"""add category parent_id (subcategories)
+
+Revision ID: 8e2f5a13c9d4
+Revises: 3b7c941bbadf
+Create Date: 2026-08-25 10:30:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '8e2f5a13c9d4'
+down_revision: Union[str, None] = '3b7c941bbadf'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('categories', sa.Column('parent_id', sa.Integer(), nullable=True))
+    op.create_foreign_key(
+        'fk_categories_parent_id_categories', 'categories', 'categories', ['parent_id'], ['id'], ondelete='SET NULL'
+    )
+    op.create_index('ix_categories_parent_id', 'categories', ['parent_id'])
+
+
+def downgrade() -> None:
+    op.drop_index('ix_categories_parent_id', table_name='categories')
+    op.drop_constraint('fk_categories_parent_id_categories', 'categories', type_='foreignkey')
+    op.drop_column('categories', 'parent_id')

backend/app/api/routes/categories.py

@@ -10,6 +10,21 @@ from app.schemas.category import CategoryCreate, CategoryRead, CategoryUpdate
 router = APIRouter(prefix="/categories", tags=["categories"])
 
 
+async def _validate_parent(session: AsyncSession, parent_id: int, kind: CategoryKind, category_id: int | None) -> None:
+    """Subcategories are one level deep only: a parent must itself be
+    top-level, and must share the child's kind (an expense category can't
+    nest under an income one, or vice versa)."""
+    if parent_id == category_id:
+        raise HTTPException(status_code=400, detail="A category cannot be its own parent")
+    parent = await session.get(Category, parent_id)
+    if parent is None:
+        raise HTTPException(status_code=400, detail="Parent category not found")
+    if parent.parent_id is not None:
+        raise HTTPException(status_code=400, detail="Subcategories can only be one level deep")
+    if parent.kind != kind:
+        raise HTTPException(status_code=400, detail="A subcategory must have the same kind as its parent")
+
+
 @router.get("", response_model=list[CategoryRead])
 async def list_categories(
     kind: CategoryKind | None = None, session: AsyncSession = Depends(get_session)
@@ -23,6 +38,8 @@ async def list_categories(
 
 @router.post("", response_model=CategoryRead, status_code=201)
 async def create_category(payload: CategoryCreate, session: AsyncSession = Depends(get_session)) -> Category:
+    if payload.parent_id is not None:
+        await _validate_parent(session, payload.parent_id, payload.kind, category_id=None)
     category = Category(**payload.model_dump(), is_default=False)
     session.add(category)
     await session.commit()
@@ -37,7 +54,15 @@ async def update_category(
     category = await session.get(Category, category_id)
     if category is None:
         raise HTTPException(status_code=404, detail="Category not found")
-    for field, value in payload.model_dump(exclude_unset=True).items():
+    updates = payload.model_dump(exclude_unset=True)
+    if "parent_id" in updates and updates["parent_id"] is not None:
+        await _validate_parent(session, updates["parent_id"], category.kind, category_id=category_id)
+        has_children = (
+            await session.execute(select(Category.id).where(Category.parent_id == category_id).limit(1))
+        ).first()
+        if has_children is not None:
+            raise HTTPException(status_code=400, detail="A category with subcategories cannot become a subcategory itself")
+    for field, value in updates.items():
         setattr(category, field, value)
     await session.commit()
     await session.refresh(category)

backend/app/models/category.py

@@ -1,5 +1,5 @@
 """A spending/income category, colored so it maps 1:1 to a dashboard chart slot."""
-from sqlalchemy import Boolean, Enum, Integer, String
+from sqlalchemy import Boolean, Enum, ForeignKey, Integer, String
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.db.base import Base
@@ -22,5 +22,12 @@ class Category(Base):
     # Fixed slot ordering keeps the donut chart's category order stable across renders.
     sort_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
     is_default: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    # Self-referential, one level deep only (routes/categories.py rejects a
+    # parent that itself has a parent) — a subcategory ("Alcohol" under
+    # "Groceries"). SET NULL so deleting a parent turns its children back
+    # into top-level categories instead of cascading the delete.
+    parent_id: Mapped[int | None] = mapped_column(ForeignKey("categories.id", ondelete="SET NULL"), nullable=True)
 
     transactions: Mapped[list["Transaction"]] = relationship(back_populates="category")
+    parent: Mapped["Category | None"] = relationship(remote_side=[id], back_populates="children")
+    children: Mapped[list["Category"]] = relationship(back_populates="parent")

backend/app/models/settings.py

@@ -2,7 +2,9 @@
 single-row table (id is always 1). Primary display currency (see UPDATES.md
 for why this doesn't do currency conversion) and the proactive-alert
 thresholds consumed by services/insights_service.py."""
-from sqlalchemy import Integer, String
+from decimal import Decimal
+
+from sqlalchemy import Integer, Numeric, String
 from sqlalchemy.orm import Mapped, mapped_column
 
 from app.db.base import Base
@@ -21,3 +23,9 @@ class AppSettings(Base):
     # tiers before insights_service.py raises risky_allocation_exceeded —
     # the classic "80% at zero risk, 20% at most exposed" rule.
     risky_allocation_threshold_percent: Mapped[int] = mapped_column(Integer, nullable=False, default=20)
+    # A depository account (checking/savings/cash) sitting at or above this
+    # balance with no transaction touching it in idle_cash_threshold_days
+    # raises insights_service.py's idle_cash alert — money that isn't
+    # working. In the app's display currency (see `currency` above).
+    idle_cash_threshold_amount: Mapped[Decimal] = mapped_column(Numeric(14, 2), nullable=False, default=Decimal("1000"))
+    idle_cash_threshold_days: Mapped[int] = mapped_column(Integer, nullable=False, default=60)

backend/app/schemas/backup.py

@@ -37,6 +37,9 @@ class CategoryBackup(BaseModel):
     color: str
     sort_order: int
     is_default: bool
+    # Defaulted so a backup exported before subcategories existed still
+    # imports cleanly under the same format version.
+    parent_id: int | None = None
 
 
 class TransactionBackup(BaseModel):
@@ -136,6 +139,10 @@ class AppSettingsBackup(BaseModel):
     # Defaulted so a backup exported before this threshold existed still
     # imports cleanly under the same format version.
     risky_allocation_threshold_percent: int = 20
+    # Defaulted so a backup exported before idle-cash thresholds existed
+    # still imports cleanly under the same format version.
+    idle_cash_threshold_amount: Decimal = Decimal("1000")
+    idle_cash_threshold_days: int = 60
 
 
 class BackupPayload(BaseModel):

backend/app/schemas/category.py

@@ -9,6 +9,9 @@ class CategoryBase(BaseModel):
     icon: str | None = None
     color: str = Field(pattern=r"^#[0-9a-fA-F]{6}$")
     sort_order: int = 0
+    # Subcategory parent, one level deep only — routes/categories.py rejects
+    # a parent that itself already has a parent.
+    parent_id: int | None = None
 
 
 class CategoryCreate(CategoryBase):
@@ -20,6 +23,10 @@ class CategoryUpdate(BaseModel):
     icon: str | None = None
     color: str | None = Field(default=None, pattern=r"^#[0-9a-fA-F]{6}$")
     sort_order: int | None = None
+    # The route uses exclude_unset=True, so sending parent_id explicitly as
+    # null (as opposed to omitting the field) is what turns a subcategory
+    # back into a top-level category.
+    parent_id: int | None = None
 
 
 class CategoryRead(CategoryBase):

backend/app/schemas/settings.py

@@ -1,3 +1,5 @@
+from decimal import Decimal
+
 from pydantic import BaseModel, Field
 
 
@@ -6,6 +8,8 @@ class AppSettingsRead(BaseModel):
     negative_cash_flow_threshold_months: int
     net_worth_decline_threshold_months: int
     risky_allocation_threshold_percent: int
+    idle_cash_threshold_amount: Decimal
+    idle_cash_threshold_days: int
 
 
 class AppSettingsUpdate(BaseModel):
@@ -24,3 +28,7 @@ class AppSettingsUpdate(BaseModel):
     # Max % of capital allowed in medium/high risk tiers (the 80/20 rule's
     # "20% at most exposed" half) before risky_allocation_exceeded fires.
     risky_allocation_threshold_percent: int | None = Field(default=None, ge=1, le=100)
+    # Balance (in the app's display currency) and days of no activity a
+    # depository account needs to hit before idle_cash fires.
+    idle_cash_threshold_amount: Decimal | None = Field(default=None, ge=0, max_digits=14, decimal_places=2)
+    idle_cash_threshold_days: int | None = Field(default=None, ge=1, le=365)

backend/app/services/backup_service.py

@@ -75,6 +75,10 @@ def _validate_references(payload: BackupPayload) -> None:
     category_ids = {row.id for row in payload.categories}
     asset_ids = {row.id for row in payload.assets}
 
+    for c in payload.categories:
+        if c.parent_id is not None and c.parent_id not in category_ids:
+            raise HTTPException(400, f"Category {c.id} references unknown parent_id {c.parent_id}")
+
     for t in payload.transactions:
         if t.account_id not in account_ids:
             raise HTTPException(400, f"Transaction {t.id} references unknown account_id {t.account_id}")
@@ -145,9 +149,14 @@ async def restore_backup(session: AsyncSession, payload: BackupPayload) -> None:
         await session.execute(delete(Category))
         await session.execute(delete(Account))
 
-        # Parents before children.
+        # Parents before children. Categories are additionally self-referential
+        # (parent_id points at another row in the same table) — sort
+        # top-level categories first so a subcategory's FK is never inserted
+        # ahead of the row it points to.
+        categories_in_order = sorted(payload.categories, key=lambda row: row.parent_id is not None)
+
         session.add_all(Account(**row.model_dump()) for row in payload.accounts)
-        session.add_all(Category(**row.model_dump()) for row in payload.categories)
+        session.add_all(Category(**row.model_dump()) for row in categories_in_order)
         session.add_all(Asset(**row.model_dump()) for row in payload.assets)
         session.add_all(Transaction(**row.model_dump()) for row in payload.transactions)
         session.add_all(AssetValuation(**row.model_dump()) for row in payload.asset_valuations)

backend/app/services/dashboard_service.py

@@ -5,6 +5,7 @@ from decimal import Decimal
 
 from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import aliased
 
 from app.models.category import Category
 from app.models.enums import TransactionType
@@ -37,22 +38,33 @@ async def get_dashboard_summary(session: AsyncSession, year: int, month: int) ->
     spent = totals.get(TransactionType.EXPENSE, Decimal("0"))
     transferred_out = totals.get(TransactionType.TRANSFER, Decimal("0"))
 
+    # A subcategory's spending rolls up into its parent's slice — Parent is a
+    # self-join to resolve each transaction's *top-level* category, falling
+    # back to the transaction's own category when it has no parent.
+    Parent = aliased(Category)
+    effective_id = func.coalesce(Category.parent_id, Category.id)
+    effective_name = func.coalesce(Parent.name, Category.name)
+    effective_color = func.coalesce(Parent.color, Category.color)
+    effective_icon = func.coalesce(Parent.icon, Category.icon)
+    effective_sort = func.coalesce(Parent.sort_order, Category.sort_order)
+
     breakdown_stmt = (
         select(
-            Category.id,
+            effective_id.label("category_id"),
-            Category.name,
+            effective_name.label("name"),
-            Category.color,
+            effective_color.label("color"),
-            Category.icon,
+            effective_icon.label("icon"),
             func.coalesce(func.sum(Transaction.amount), 0).label("amount"),
         )
         .join(Category, Category.id == Transaction.category_id)
+        .outerjoin(Parent, Parent.id == Category.parent_id)
         .where(
             Transaction.date >= start,
             Transaction.date <= end,
             Transaction.type == TransactionType.EXPENSE,
         )
-        .group_by(Category.id, Category.name, Category.color, Category.icon, Category.sort_order)
+        .group_by(effective_id, effective_name, effective_color, effective_icon, effective_sort)
-        .order_by(func.sum(Transaction.amount).desc(), Category.sort_order)
+        .order_by(func.sum(Transaction.amount).desc(), effective_sort)
     )
     breakdown_result = await session.execute(breakdown_stmt)
     rows = breakdown_result.all()

backend/app/services/insights_service.py

@@ -1,20 +1,26 @@
 """Proactive early-warning checks, computed from data that already exists.
-Four signals: sustained negative monthly cash flow, a sustained net-worth
+Five signals: sustained negative monthly cash flow, a sustained net-worth
-decline, one or more over-budget categories, and too much capital in
+decline, one or more over-budget categories, too much capital in medium/high
-medium/high risk tiers (the "80% at zero risk, 20% at most exposed" rule).
+risk tiers (the "80% at zero risk, 20% at most exposed" rule), and cash
-The first two only look at fully-elapsed calendar months, so a same-month
+sitting idle in a depository account. The first two only look at
-false alarm (rent already paid, salary not landed yet) never fires — their
+fully-elapsed calendar months, so a same-month false alarm (rent already
-"sustained for how long" thresholds, and the risk-allocation percentage, are
+paid, salary not landed yet) never fires — their "sustained for how long"
+thresholds, the risk-allocation percentage, and the idle-cash amount/days are
 all user-configurable (Settings page), stored on AppSettings, see
 get_or_create_app_settings. The budget check is the opposite: it deliberately
 looks at the CURRENT, still-in-progress month, since the whole point is to
 catch overspending while there's still time to react.
 """
-from datetime import date
+from collections import defaultdict
+from datetime import date, timedelta
 from decimal import Decimal
 
+from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from app.models.account import Account
+from app.models.enums import AccountType, TransactionType
+from app.models.transaction import Transaction
 from app.schemas.insights import AlertsResponse, FinancialAlert
 from app.schemas.net_worth import NetWorthSummary
 from app.services.budget_service import get_budget_status
@@ -24,6 +30,11 @@ from app.services.settings_service import get_or_create_app_settings
 
 MAX_LOOKBACK_MONTHS = 24
 
+# Accounts where a big, untouched balance means money isn't working — a
+# credit card balance isn't "your cash", an investment account is already
+# invested, and OTHER is too ambiguous to guess at.
+_IDLE_CASH_ACCOUNT_TYPES = (AccountType.CHECKING, AccountType.SAVINGS, AccountType.CASH)
+
 
 def _previous_month(year: int, month: int) -> tuple[int, int]:
     return (year - 1, 12) if month == 1 else (year, month - 1)
@@ -68,6 +79,55 @@ def _net_worth_decline_streak(summary: NetWorthSummary) -> int:
     return streak
 
 
+async def _idle_cash_account_count(session: AsyncSession, threshold_amount: Decimal, threshold_days: int) -> int:
+    eligible_ids = set(
+        (
+            await session.execute(
+                select(Account.id).where(
+                    Account.is_archived.is_(False), Account.type.in_(_IDLE_CASH_ACCOUNT_TYPES)
+                )
+            )
+        )
+        .scalars()
+        .all()
+    )
+    if not eligible_ids:
+        return 0
+
+    # Same derivation account_service._account_balances uses — balance is
+    # never stored, only ever summed from the full transaction history — plus
+    # tracking the most recent date that touched each account along the way.
+    rows = await session.execute(
+        select(Transaction.type, Transaction.amount, Transaction.account_id, Transaction.transfer_account_id, Transaction.date)
+    )
+    balances: dict[int, Decimal] = defaultdict(Decimal)
+    last_activity: dict[int, date] = {}
+
+    def touch(account_id: int | None, tx_date: date) -> None:
+        if account_id in eligible_ids and (account_id not in last_activity or tx_date > last_activity[account_id]):
+            last_activity[account_id] = tx_date
+
+    for tx_type, amount, account_id, transfer_account_id, tx_date in rows.all():
+        if tx_type == TransactionType.INCOME:
+            balances[account_id] += amount
+        elif tx_type == TransactionType.EXPENSE:
+            balances[account_id] -= amount
+        elif tx_type == TransactionType.TRANSFER:
+            balances[account_id] -= amount
+            if transfer_account_id is not None:
+                balances[transfer_account_id] += amount
+        touch(account_id, tx_date)
+        touch(transfer_account_id, tx_date)
+
+    cutoff = date.today() - timedelta(days=threshold_days)
+    return sum(
+        1
+        for account_id in eligible_ids
+        if balances.get(account_id, Decimal("0")) >= threshold_amount
+        and last_activity.get(account_id, cutoff) <= cutoff
+    )
+
+
 async def get_financial_alerts(session: AsyncSession) -> AlertsResponse:
     settings = await get_or_create_app_settings(session)
     alerts: list[FinancialAlert] = []
@@ -116,4 +176,16 @@ async def get_financial_alerts(session: AsyncSession) -> AlertsResponse:
             )
         )
 
+    idle_cash_count = await _idle_cash_account_count(
+        session, settings.idle_cash_threshold_amount, settings.idle_cash_threshold_days
+    )
+    if idle_cash_count > 0:
+        alerts.append(
+            FinancialAlert(
+                key="idle_cash",
+                severity="warning",
+                params={"count": idle_cash_count, "days": settings.idle_cash_threshold_days},
+            )
+        )
+
     return AlertsResponse(alerts=alerts)

backend/app/services/reports_service.py

@@ -10,6 +10,7 @@ from decimal import Decimal
 from fastapi import HTTPException
 from sqlalchemy import extract, func, select
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import aliased
 
 from app.models.category import Category
 from app.models.enums import CategoryKind, TransactionType
@@ -33,8 +34,18 @@ async def get_category_spending_report(
     if category is None:
         raise HTTPException(status_code=404, detail="Category not found")
 
+    # A top-level category's own report folds in its subcategories' spending
+    # too (same rollup as the Dashboard breakdown); a subcategory picked
+    # directly shows just its own transactions — there's nothing beneath it.
+    category_ids: list[int] = [category_id]
+    if category.parent_id is None:
+        child_ids = (
+            await session.execute(select(Category.id).where(Category.parent_id == category_id))
+        ).scalars().all()
+        category_ids.extend(child_ids)
+
     bounds_stmt = select(func.min(Transaction.date), func.max(Transaction.date)).where(
-        Transaction.category_id == category_id
+        Transaction.category_id.in_(category_ids)
     )
     if start_date:
         bounds_stmt = bounds_stmt.where(Transaction.date >= start_date)
@@ -68,7 +79,7 @@ async def get_category_spending_report(
             func.count(Transaction.id).label("cnt"),
         )
         .where(
-            Transaction.category_id == category_id,
+            Transaction.category_id.in_(category_ids),
             Transaction.date >= effective_start,
             Transaction.date <= effective_end,
         )
@@ -115,24 +126,34 @@ async def get_category_ranking_report(
     arbitrary period — "which category costs the most" across the whole
     range, unlike the month-scoped Dashboard breakdown or the
     single-category detail above."""
+    # Same subcategory-into-parent rollup as the Dashboard breakdown — see
+    # dashboard_service.get_dashboard_summary.
+    Parent = aliased(Category)
+    effective_id = func.coalesce(Category.parent_id, Category.id)
+    effective_name = func.coalesce(Parent.name, Category.name)
+    effective_color = func.coalesce(Parent.color, Category.color)
+    effective_icon = func.coalesce(Parent.icon, Category.icon)
+    effective_sort = func.coalesce(Parent.sort_order, Category.sort_order)
+
     stmt = (
         select(
-            Category.id,
+            effective_id.label("category_id"),
-            Category.name,
+            effective_name.label("name"),
-            Category.color,
+            effective_color.label("color"),
-            Category.icon,
+            effective_icon.label("icon"),
             func.sum(Transaction.amount).label("amount"),
             func.count(Transaction.id).label("cnt"),
         )
         .join(Category, Category.id == Transaction.category_id)
+        .outerjoin(Parent, Parent.id == Category.parent_id)
         .where(Category.kind == kind, Transaction.type == _KIND_TO_TRANSACTION_TYPE[kind])
     )
     if start_date:
         stmt = stmt.where(Transaction.date >= start_date)
     if end_date:
         stmt = stmt.where(Transaction.date <= end_date)
-    stmt = stmt.group_by(Category.id, Category.name, Category.color, Category.icon, Category.sort_order)
+    stmt = stmt.group_by(effective_id, effective_name, effective_color, effective_icon, effective_sort)
-    stmt = stmt.order_by(func.sum(Transaction.amount).desc(), Category.sort_order)
+    stmt = stmt.order_by(func.sum(Transaction.amount).desc(), effective_sort)
 
     rows = (await session.execute(stmt)).all()
     total_amount = sum((row.amount for row in rows), Decimal("0"))

backend/tests/test_categories.py

@@ -0,0 +1,69 @@
+"""Category subcategories (parent_id): creation rules and the one-level-deep
+invariant enforced in api/routes/categories.py.
+"""
+from httpx import AsyncClient
+
+
+async def test_create_subcategory_under_top_level_category(client: AsyncClient, categories):
+    groceries = categories["Groceries"]["id"]
+    resp = await client.post(
+        "/categories", json={"name": "Alcohol", "kind": "expense", "color": "#e34948", "parent_id": groceries}
+    )
+    assert resp.status_code == 201
+    assert resp.json()["parent_id"] == groceries
+
+
+async def test_subcategory_cannot_have_a_different_kind_than_its_parent(client: AsyncClient, categories):
+    groceries = categories["Groceries"]["id"]
+    resp = await client.post(
+        "/categories", json={"name": "Bad", "kind": "income", "color": "#e34948", "parent_id": groceries}
+    )
+    assert resp.status_code == 400
+
+
+async def test_subcategories_are_only_one_level_deep(client: AsyncClient, categories):
+    groceries = categories["Groceries"]["id"]
+    child = await client.post(
+        "/categories", json={"name": "Alcohol", "kind": "expense", "color": "#e34948", "parent_id": groceries}
+    )
+    child_id = child.json()["id"]
+
+    resp = await client.post(
+        "/categories", json={"name": "Wine", "kind": "expense", "color": "#e34948", "parent_id": child_id}
+    )
+    assert resp.status_code == 400
+
+
+async def test_category_cannot_be_its_own_parent(client: AsyncClient, categories):
+    groceries = categories["Groceries"]["id"]
+    resp = await client.patch(f"/categories/{groceries}", json={"parent_id": groceries})
+    assert resp.status_code == 400
+
+
+async def test_a_category_with_subcategories_cannot_itself_become_a_subcategory(client: AsyncClient, categories):
+    groceries = categories["Groceries"]["id"]
+    dining = categories["Dining Out"]["id"]
+    await client.post(
+        "/categories", json={"name": "Alcohol", "kind": "expense", "color": "#e34948", "parent_id": groceries}
+    )
+
+    resp = await client.patch(f"/categories/{groceries}", json={"parent_id": dining})
+    assert resp.status_code == 400
+
+
+async def test_deleting_a_parent_leaves_children_as_top_level(client: AsyncClient, categories):
+    # A default category can't be deleted at all (separate rule) — use a
+    # fresh, non-default parent so this test isolates the parent/child rule.
+    parent = await client.post("/categories", json={"name": "Custom Parent", "kind": "expense", "color": "#e34948"})
+    parent_id = parent.json()["id"]
+    created = await client.post(
+        "/categories", json={"name": "Alcohol", "kind": "expense", "color": "#e34948", "parent_id": parent_id}
+    )
+    child_id = created.json()["id"]
+
+    resp = await client.delete(f"/categories/{parent_id}")
+    assert resp.status_code == 204
+
+    refetched = await client.get("/categories")
+    child = next(c for c in refetched.json() if c["id"] == child_id)
+    assert child["parent_id"] is None

backend/tests/test_dashboard.py

@@ -81,6 +81,23 @@ async def test_spending_by_category_amounts_and_percent(client: AsyncClient, acc
     assert breakdown["Dining Out"]["percent"] == 25.0
 
 
+async def test_subcategory_spending_rolls_up_into_its_parent(client: AsyncClient, account_id, categories):
+    groceries = categories["Groceries"]["id"]
+    alcohol = await client.post(
+        "/categories", json={"name": "Alcohol", "kind": "expense", "color": "#e34948", "parent_id": groceries}
+    )
+    alcohol_id = alcohol.json()["id"]
+
+    await client.post("/transactions", json=_txn(account_id, amount="60.00", category_id=groceries, date="2026-08-01"))
+    await client.post("/transactions", json=_txn(account_id, amount="15.00", category_id=alcohol_id, date="2026-08-02"))
+
+    resp = await client.get("/dashboard/summary", params={"year": 2026, "month": 8})
+    breakdown = {row["name"]: row for row in resp.json()["spending_by_category"]}
+
+    assert "Alcohol" not in breakdown
+    assert money(breakdown["Groceries"]["amount"]) == Decimal("75.00")
+
+
 async def test_more_than_eight_expense_categories_roll_up_into_other(client: AsyncClient, account_id, categories):
     expense_categories = [c for c in categories.values() if c["kind"] == "expense"]
     # The default seed only ships 8 expense categories (see app/db/seed.py) —

backend/tests/test_insights.py

@@ -0,0 +1,64 @@
+"""Proactive alerts (services/insights_service.py): only the idle-cash
+signal is covered here — the other three (negative cash flow, net worth
+decline, over-budget) predate this test file and aren't touched by this
+change.
+"""
+from datetime import date, timedelta
+
+from httpx import AsyncClient
+
+from tests.helpers import txn_payload as _txn
+
+STALE_DATE = (date.today() - timedelta(days=400)).isoformat()
+RECENT_DATE = date.today().isoformat()
+
+
+async def _alert_keys(client: AsyncClient) -> set[str]:
+    resp = await client.get("/insights/alerts")
+    return {alert["key"] for alert in resp.json()["alerts"]}
+
+
+async def test_idle_cash_flags_a_stale_large_balance(client: AsyncClient, account_id, categories):
+    await client.post(
+        "/transactions",
+        json=_txn(account_id, type="income", amount="5000.00", category_id=categories["Salary"]["id"], date=STALE_DATE),
+    )
+
+    assert "idle_cash" in await _alert_keys(client)
+
+
+async def test_idle_cash_ignores_a_recently_touched_account(client: AsyncClient, account_id, categories):
+    await client.post(
+        "/transactions",
+        json=_txn(account_id, type="income", amount="5000.00", category_id=categories["Salary"]["id"], date=STALE_DATE),
+    )
+    await client.post(
+        "/transactions",
+        json=_txn(account_id, type="expense", amount="1.00", category_id=categories["Groceries"]["id"], date=RECENT_DATE),
+    )
+
+    assert "idle_cash" not in await _alert_keys(client)
+
+
+async def test_idle_cash_ignores_investment_accounts(client: AsyncClient, categories):
+    investment = await client.post("/accounts", json={"name": "Brokerage", "type": "investment", "currency": "USD"})
+    investment_id = investment.json()["id"]
+    await client.post(
+        "/transactions",
+        json=_txn(investment_id, type="income", amount="5000.00", category_id=categories["Salary"]["id"], date=STALE_DATE),
+    )
+
+    assert "idle_cash" not in await _alert_keys(client)
+
+
+async def test_idle_cash_respects_custom_threshold(client: AsyncClient, account_id, categories):
+    await client.post(
+        "/transactions",
+        json=_txn(account_id, type="income", amount="5000.00", category_id=categories["Salary"]["id"], date=STALE_DATE),
+    )
+    assert "idle_cash" in await _alert_keys(client)
+
+    resp = await client.patch("/settings", json={"idle_cash_threshold_amount": "10000.00"})
+    assert resp.status_code == 200
+
+    assert "idle_cash" not in await _alert_keys(client)

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "aurum-frontend",
   "private": true,
-  "version": "0.23.2",
+  "version": "0.24.0",
   "type": "module",
   "scripts": {
     "dev": "vite",

frontend/src/components/categories/CategoryFormModal.tsx

@@ -4,7 +4,8 @@ import { Button } from "@/components/ui/Button";
 import { Input, Label, Select } from "@/components/ui/Input";
 import { CategoryColorPicker } from "@/components/categories/CategoryColorPicker";
 import { CategoryIconPicker } from "@/components/categories/CategoryIconPicker";
-import { useCreateCategory, useUpdateCategory } from "@/hooks/useCategories";
+import { useCategories, useCreateCategory, useUpdateCategory } from "@/hooks/useCategories";
+import { translateCategoryName } from "@/lib/categoryLabels";
 import { useTranslation, type TranslationKey } from "@/lib/i18n";
 import type { Category, CategoryKind } from "@/types";
 
@@ -20,11 +21,12 @@ interface CategoryFormModalProps {
 const KINDS: CategoryKind[] = ["expense", "income"];
 
 function emptyForm(kind: CategoryKind) {
-  return { name: "", kind, icon: "wallet", color: "#2a78d6" };
+  return { name: "", kind, icon: "wallet", color: "#2a78d6", parent_id: "" };
 }
 
 export function CategoryFormModal({ open, onClose, category, defaultKind }: CategoryFormModalProps) {
   const { t } = useTranslation();
+  const { data: allCategories } = useCategories();
   const createCategory = useCreateCategory();
   const updateCategory = useUpdateCategory();
 
@@ -35,26 +37,41 @@ export function CategoryFormModal({ open, onClose, category, defaultKind }: Cate
     if (!open) return;
     setForm(
       category
-        ? { name: category.name, kind: category.kind, icon: category.icon ?? "wallet", color: category.color }
+        ? {
+            name: category.name,
+            kind: category.kind,
+            icon: category.icon ?? "wallet",
+            color: category.color,
+            parent_id: category.parent_id ? String(category.parent_id) : "",
+          }
         : emptyForm(defaultKind)
     );
     setError(null);
   }, [open, category, defaultKind]);
 
+  // A category that already has subcategories can't become one itself
+  // (backend rejects it) — the parent picker is disabled in that case.
+  const hasChildren = category ? (allCategories ?? []).some((c) => c.parent_id === category.id) : false;
+  const parentCandidates = (allCategories ?? []).filter(
+    (c) => c.kind === form.kind && c.parent_id === null && c.id !== category?.id
+  );
+
   const isSaving = createCategory.isPending || updateCategory.isPending;
 
   async function handleSubmit(event: React.FormEvent) {
     event.preventDefault();
     setError(null);
 
+    const parent_id = form.parent_id ? Number(form.parent_id) : null;
+
     try {
       if (category) {
         await updateCategory.mutateAsync({
           id: category.id,
-          input: { name: form.name, icon: form.icon, color: form.color },
+          input: { name: form.name, icon: form.icon, color: form.color, parent_id },
         });
       } else {
-        await createCategory.mutateAsync(form);
+        await createCategory.mutateAsync({ name: form.name, kind: form.kind, icon: form.icon, color: form.color, parent_id });
       }
       onClose();
     } catch {
@@ -82,7 +99,9 @@ export function CategoryFormModal({ open, onClose, category, defaultKind }: Cate
             id="category-kind"
             value={form.kind}
             disabled={Boolean(category)}
-            onChange={(event) => setForm((prev) => ({ ...prev, kind: event.target.value as CategoryKind }))}
+            onChange={(event) =>
+              setForm((prev) => ({ ...prev, kind: event.target.value as CategoryKind, parent_id: "" }))
+            }
           >
             {KINDS.map((kind) => (
               <option key={kind} value={kind}>
@@ -92,6 +111,24 @@ export function CategoryFormModal({ open, onClose, category, defaultKind }: Cate
           </Select>
         </div>
 
+        <div>
+          <Label htmlFor="category-parent">{t("category.form.parentLabel")}</Label>
+          <Select
+            id="category-parent"
+            value={hasChildren ? "" : form.parent_id}
+            disabled={hasChildren}
+            onChange={(event) => setForm((prev) => ({ ...prev, parent_id: event.target.value }))}
+          >
+            <option value="">{t("category.form.noParent")}</option>
+            {parentCandidates.map((parent) => (
+              <option key={parent.id} value={parent.id}>
+                {translateCategoryName(parent.name)}
+              </option>
+            ))}
+          </Select>
+          {hasChildren && <p className="mt-1 text-xs text-text-muted">{t("category.form.hasChildrenHint")}</p>}
+        </div>
+
         <div>
           <Label>{t("category.form.iconLabel")}</Label>
           <CategoryIconPicker value={form.icon} onChange={(icon) => setForm((prev) => ({ ...prev, icon }))} />

frontend/src/components/categories/CategoryList.tsx

@@ -10,6 +10,57 @@ interface CategoryListProps {
   onDelete: (category: Category) => void;
 }
 
+interface RowProps {
+  category: Category;
+  indented: boolean;
+  onEdit: (category: Category) => void;
+  onDelete: (category: Category) => void;
+}
+
+function CategoryRow({ category, indented, onEdit, onDelete }: RowProps) {
+  const { t } = useTranslation();
+  const Icon = getCategoryIcon(category.icon);
+
+  return (
+    <li className={`flex items-center gap-3 py-2.5 ${indented ? "pl-8" : ""}`}>
+      <span
+        className="flex h-8 w-8 shrink-0 items-center justify-center rounded-full"
+        style={{ backgroundColor: `${category.color}1a`, color: category.color }}
+      >
+        <Icon size={15} />
+      </span>
+      <span className="flex min-w-0 flex-1 items-center gap-1.5 truncate text-sm font-medium text-text-primary">
+        <span className="truncate">{translateCategoryName(category.name)}</span>
+        {category.is_default && (
+          <span className="shrink-0 rounded bg-surface-2 px-1 py-0.5 text-[10px] leading-none text-text-muted">
+            {t("category.defaultBadge")}
+          </span>
+        )}
+      </span>
+      <span className="flex shrink-0 gap-1">
+        <button
+          type="button"
+          aria-label={t("common.edit")}
+          onClick={() => onEdit(category)}
+          className="rounded-md p-1.5 text-text-muted hover:bg-surface-2 hover:text-text-primary"
+        >
+          <Pencil size={15} />
+        </button>
+        <button
+          type="button"
+          aria-label={t("common.delete")}
+          title={category.is_default ? t("category.defaultCannotDelete") : undefined}
+          disabled={category.is_default}
+          onClick={() => onDelete(category)}
+          className="rounded-md p-1.5 text-text-muted hover:bg-surface-2 hover:text-danger disabled:cursor-not-allowed disabled:opacity-30 disabled:hover:bg-transparent disabled:hover:text-text-muted"
+        >
+          <Trash2 size={15} />
+        </button>
+      </span>
+    </li>
+  );
+}
+
 export function CategoryList({ items, onEdit, onDelete }: CategoryListProps) {
   const { t } = useTranslation();
 
@@ -17,50 +68,31 @@ export function CategoryList({ items, onEdit, onDelete }: CategoryListProps) {
     return <p className="py-6 text-center text-sm text-text-muted">{t("category.empty")}</p>;
   }
 
+  // `items` arrives pre-sorted (alphabetically, by CategoriesPage) — walk
+  // top-level categories in that order and splice each one's children in
+  // directly beneath it, so a subcategory renders indented under its parent
+  // while everything stays a single flat, evenly-divided list.
+  const topLevel = items.filter((category) => category.parent_id === null);
+  const topLevelIds = new Set(topLevel.map((category) => category.id));
+  // Defensive only — a child whose parent got filtered out (e.g. a kind
+  // mismatch) should never happen given the create/update validation, but
+  // falls back to a top-level row instead of silently disappearing.
+  const orphaned = items.filter((category) => category.parent_id !== null && !topLevelIds.has(category.parent_id));
+
+  const rows: { category: Category; indented: boolean }[] = [];
+  for (const parent of topLevel) {
+    rows.push({ category: parent, indented: false });
+    for (const child of items) {
+      if (child.parent_id === parent.id) rows.push({ category: child, indented: true });
+    }
+  }
+  for (const category of orphaned) rows.push({ category, indented: false });
+
   return (
     <ul className="divide-y divide-gridline">
-      {items.map((category) => {
+      {rows.map(({ category, indented }) => (
-        const Icon = getCategoryIcon(category.icon);
+        <CategoryRow key={category.id} category={category} indented={indented} onEdit={onEdit} onDelete={onDelete} />
-
+      ))}
-        return (
-          <li key={category.id} className="flex items-center gap-3 py-2.5">
-            <span
-              className="flex h-8 w-8 shrink-0 items-center justify-center rounded-full"
-              style={{ backgroundColor: `${category.color}1a`, color: category.color }}
-            >
-              <Icon size={15} />
-            </span>
-            <span className="flex min-w-0 flex-1 items-center gap-1.5 truncate text-sm font-medium text-text-primary">
-              <span className="truncate">{translateCategoryName(category.name)}</span>
-              {category.is_default && (
-                <span className="shrink-0 rounded bg-surface-2 px-1 py-0.5 text-[10px] leading-none text-text-muted">
-                  {t("category.defaultBadge")}
-                </span>
-              )}
-            </span>
-            <span className="flex shrink-0 gap-1">
-              <button
-                type="button"
-                aria-label={t("common.edit")}
-                onClick={() => onEdit(category)}
-                className="rounded-md p-1.5 text-text-muted hover:bg-surface-2 hover:text-text-primary"
-              >
-                <Pencil size={15} />
-              </button>
-              <button
-                type="button"
-                aria-label={t("common.delete")}
-                title={category.is_default ? t("category.defaultCannotDelete") : undefined}
-                disabled={category.is_default}
-                onClick={() => onDelete(category)}
-                className="rounded-md p-1.5 text-text-muted hover:bg-surface-2 hover:text-danger disabled:cursor-not-allowed disabled:opacity-30 disabled:hover:bg-transparent disabled:hover:text-text-muted"
-              >
-                <Trash2 size={15} />
-              </button>
-            </span>
-          </li>
-        );
-      })}
     </ul>
   );
 }

frontend/src/components/insights/AlertBanner.tsx

@@ -9,6 +9,7 @@ const ALERT_MESSAGE_KEYS: Record<string, TranslationKey> = {
   net_worth_decline_streak: "insights.netWorthDecline",
   budget_exceeded: "insights.budgetExceeded",
   risky_allocation_exceeded: "insights.riskyAllocationExceeded",
+  idle_cash: "insights.idleCash",
 };
 
 function alertMessage(alert: FinancialAlert, t: ReturnType<typeof useTranslation>["t"]): string {
@@ -18,6 +19,9 @@ function alertMessage(alert: FinancialAlert, t: ReturnType<typeof useTranslation
   if (alert.key === "budget_exceeded" && alert.params.count === 1) {
     return t("insights.budgetExceededOne");
   }
+  if (alert.key === "idle_cash" && alert.params.count === 1) {
+    return t("insights.idleCashOne", alert.params);
+  }
   const messageKey = ALERT_MESSAGE_KEYS[alert.key];
   if (!messageKey) return alert.key;
   return t(messageKey, alert.params);

frontend/src/components/settings/AlertThresholdsCard.tsx

@@ -8,6 +8,9 @@ const MIN_MONTHS = 1;
 const MAX_MONTHS = 24;
 const MIN_PERCENT = 1;
 const MAX_PERCENT = 100;
+const MIN_AMOUNT = 0;
+const MIN_DAYS = 1;
+const MAX_DAYS = 365;
 
 function clamp(value: number, min: number, max: number): number {
   return Math.min(max, Math.max(min, value));
@@ -21,12 +24,16 @@ export function AlertThresholdsCard() {
   const [cashFlowMonths, setCashFlowMonths] = useState("");
   const [netWorthMonths, setNetWorthMonths] = useState("");
   const [riskyPercent, setRiskyPercent] = useState("");
+  const [idleCashAmount, setIdleCashAmount] = useState("");
+  const [idleCashDays, setIdleCashDays] = useState("");
 
   useEffect(() => {
     if (!settings) return;
     setCashFlowMonths(String(settings.negative_cash_flow_threshold_months));
     setNetWorthMonths(String(settings.net_worth_decline_threshold_months));
     setRiskyPercent(String(settings.risky_allocation_threshold_percent));
+    setIdleCashAmount(settings.idle_cash_threshold_amount);
+    setIdleCashDays(String(settings.idle_cash_threshold_days));
   }, [settings]);
 
   function commitCashFlowMonths() {
@@ -53,13 +60,29 @@ export function AlertThresholdsCard() {
     }
   }
 
+  function commitIdleCashAmount() {
+    const value = Math.max(MIN_AMOUNT, Number(idleCashAmount) || MIN_AMOUNT).toFixed(2);
+    setIdleCashAmount(value);
+    if (settings && value !== settings.idle_cash_threshold_amount) {
+      updateSettings.mutate({ idle_cash_threshold_amount: value });
+    }
+  }
+
+  function commitIdleCashDays() {
+    const value = clamp(Number(idleCashDays) || MIN_DAYS, MIN_DAYS, MAX_DAYS);
+    setIdleCashDays(String(value));
+    if (settings && value !== settings.idle_cash_threshold_days) {
+      updateSettings.mutate({ idle_cash_threshold_days: value });
+    }
+  }
+
   return (
     <Card>
       <CardHeader>
         <CardTitle>{t("settings.alertsTitle")}</CardTitle>
       </CardHeader>
       <CardContent className="space-y-3">
-        <div className="grid grid-cols-1 gap-3 sm:grid-cols-3">
+        <div className="grid grid-cols-1 gap-3 sm:grid-cols-2 lg:grid-cols-3">
           <div>
             <Label htmlFor="cash-flow-threshold">{t("settings.negativeCashFlowThresholdLabel")}</Label>
             <Input
@@ -99,6 +122,31 @@ export function AlertThresholdsCard() {
               onBlur={commitRiskyPercent}
             />
           </div>
+          <div>
+            <Label htmlFor="idle-cash-amount-threshold">{t("settings.idleCashThresholdAmountLabel")}</Label>
+            <Input
+              id="idle-cash-amount-threshold"
+              type="number"
+              min={MIN_AMOUNT}
+              step={0.01}
+              value={idleCashAmount}
+              onChange={(event) => setIdleCashAmount(event.target.value)}
+              onBlur={commitIdleCashAmount}
+            />
+          </div>
+          <div>
+            <Label htmlFor="idle-cash-days-threshold">{t("settings.idleCashThresholdDaysLabel")}</Label>
+            <Input
+              id="idle-cash-days-threshold"
+              type="number"
+              min={MIN_DAYS}
+              max={MAX_DAYS}
+              step={1}
+              value={idleCashDays}
+              onChange={(event) => setIdleCashDays(event.target.value)}
+              onBlur={commitIdleCashDays}
+            />
+          </div>
         </div>
         <p className="text-xs text-text-muted">{t("settings.alertsHint")}</p>
       </CardContent>

frontend/src/components/transactions/TransactionFormModal.tsx

@@ -7,7 +7,7 @@ import { useCategories } from "@/hooks/useCategories";
 import { useCreateTransaction, useUpdateTransaction } from "@/hooks/useTransactions";
 import { useTranslation } from "@/lib/i18n";
 import { translateCategoryName } from "@/lib/categoryLabels";
-import type { Transaction, TransactionInput, TransactionType } from "@/types";
+import type { Category, Transaction, TransactionInput, TransactionType } from "@/types";
 
 interface TransactionFormModalProps {
   open: boolean;
@@ -32,7 +32,7 @@ const EMPTY_FORM = {
 };
 
 export function TransactionFormModal({ open, onClose, transaction }: TransactionFormModalProps) {
-  const { t } = useTranslation();
+  const { t, language } = useTranslation();
   const { data: accounts } = useAccounts();
   const { data: categories } = useCategories();
   const createTransaction = useCreateTransaction();
@@ -61,9 +61,19 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
     setError(null);
   }, [open, transaction, accounts]);
 
-  const relevantCategories = (categories ?? []).filter((category) =>
+  // Subcategories are listed right under their parent (not scattered by
-    form.type === "income" ? category.kind === "income" : category.kind === "expense"
+  // sort_order) so the hierarchy set up on the Categories page reads the
-  );
+  // same way here.
+  const kindCategories = (categories ?? [])
+    .filter((category) => (form.type === "income" ? category.kind === "income" : category.kind === "expense"))
+    .sort((a, b) => translateCategoryName(a.name).localeCompare(translateCategoryName(b.name), language));
+  const relevantCategories: (Category & { indented?: boolean })[] = [];
+  for (const parent of kindCategories.filter((category) => category.parent_id === null)) {
+    relevantCategories.push(parent);
+    for (const child of kindCategories.filter((category) => category.parent_id === parent.id)) {
+      relevantCategories.push({ ...child, indented: true });
+    }
+  }
 
   const isSaving = createTransaction.isPending || updateTransaction.isPending;
 
@@ -217,6 +227,7 @@ export function TransactionFormModal({ open, onClose, transaction }: Transaction
               <option value="">{t("transactions.form.noCategory")}</option>
               {relevantCategories.map((category) => (
                 <option key={category.id} value={category.id}>
+                  {category.indented ? `    ↳ ` : ""}
                   {translateCategoryName(category.name)}
                 </option>
               ))}

frontend/src/lib/i18n.ts

@@ -203,6 +203,8 @@ const ru = {
   "settings.negativeCashFlowThresholdLabel": "Расходы превышают доходы, мес. подряд",
   "settings.netWorthDeclineThresholdLabel": "Капитал снижается, мес. подряд",
   "settings.riskyAllocationThresholdLabel": "Допустимая доля рискового капитала, %",
+  "settings.idleCashThresholdAmountLabel": "Простаивающий кэш — сумма от",
+  "settings.idleCashThresholdDaysLabel": "Простаивающий кэш — без движения, дней",
 
   "backup.title": "Резервное копирование",
   "backup.description":
@@ -226,6 +228,8 @@ const ru = {
   "insights.budgetExceededOne": "Превышен бюджет в 1 категории в этом месяце",
   "insights.budgetExceeded": "Превышен бюджет в {{count}} категориях в этом месяце",
   "insights.riskyAllocationExceeded": "Рискованная часть капитала — {{percent}}%, превышает установленный лимит {{threshold}}%",
+  "insights.idleCashOne": "На 1 счёте лежит крупная сумма без движения {{days}}+ дней — возможно, эти деньги стоит куда-то вложить",
+  "insights.idleCash": "На {{count}} счетах лежит крупная сумма без движения {{days}}+ дней — возможно, эти деньги стоит куда-то вложить",
 
   "budget.empty": "Бюджетов пока нет. Задайте лимит для категории расходов, чтобы отслеживать её здесь.",
   "budget.remaining": "{{amount}} осталось",
@@ -314,6 +318,9 @@ const ru = {
   "category.form.nameLabel": "Название",
   "category.form.namePlaceholder": "Продукты, транспорт, подписки…",
   "category.form.kindLabel": "Тип",
+  "category.form.parentLabel": "Родительская категория",
+  "category.form.noParent": "— без родителя (верхний уровень) —",
+  "category.form.hasChildrenHint": "У категории уже есть подкатегории — она не может сама стать подкатегорией.",
   "category.form.iconLabel": "Иконка",
   "category.form.colorLabel": "Цвет",
   "category.form.customColorLabel": "Свой цвет",
@@ -559,6 +566,8 @@ const en: Record<keyof typeof ru, string> = {
   "settings.negativeCashFlowThresholdLabel": "Expenses exceed income, consecutive months",
   "settings.netWorthDeclineThresholdLabel": "Net worth declining, consecutive months",
   "settings.riskyAllocationThresholdLabel": "Max % of capital allowed at risk",
+  "settings.idleCashThresholdAmountLabel": "Idle cash — balance at or above",
+  "settings.idleCashThresholdDaysLabel": "Idle cash — no activity for, days",
 
   "backup.title": "Backup & Restore",
   "backup.description":
@@ -582,6 +591,8 @@ const en: Record<keyof typeof ru, string> = {
   "insights.budgetExceededOne": "Budget exceeded in 1 category this month",
   "insights.budgetExceeded": "Budget exceeded in {{count}} categories this month",
   "insights.riskyAllocationExceeded": "Risky share of capital is {{percent}}%, above your {{threshold}}% limit",
+  "insights.idleCashOne": "1 account is holding a large balance with no activity for {{days}}+ days — that money might be worth putting to work",
+  "insights.idleCash": "{{count}} accounts are holding a large balance with no activity for {{days}}+ days — that money might be worth putting to work",
 
   "budget.empty": "No budgets yet. Set a limit for an expense category to start tracking it here.",
   "budget.remaining": "{{amount}} left",
@@ -670,6 +681,9 @@ const en: Record<keyof typeof ru, string> = {
   "category.form.nameLabel": "Name",
   "category.form.namePlaceholder": "Groceries, transport, subscriptions…",
   "category.form.kindLabel": "Type",
+  "category.form.parentLabel": "Parent category",
+  "category.form.noParent": "— no parent (top level) —",
+  "category.form.hasChildrenHint": "This category already has subcategories — it can't become a subcategory itself.",
   "category.form.iconLabel": "Icon",
   "category.form.colorLabel": "Color",
   "category.form.customColorLabel": "Custom color",

frontend/src/types/index.ts

@@ -34,6 +34,9 @@ export interface Category {
   color: string;
   sort_order: number;
   is_default: boolean;
+  // Subcategory parent, one level deep only (a category whose own parent_id
+  // is set can't itself have children — enforced backend-side).
+  parent_id: number | null;
 }
 
 export interface CategoryInput {
@@ -42,6 +45,7 @@ export interface CategoryInput {
   icon?: string | null;
   color: string;
   sort_order?: number;
+  parent_id?: number | null;
 }
 
 // kind is fixed at creation on the backend (CategoryUpdate has no kind field).
@@ -50,6 +54,7 @@ export interface CategoryUpdateInput {
   icon?: string | null;
   color?: string;
   sort_order?: number;
+  parent_id?: number | null;
 }
 
 export interface Transaction {
@@ -365,5 +370,7 @@ export interface AppSettings {
   negative_cash_flow_threshold_months: number;
   net_worth_decline_threshold_months: number;
   risky_allocation_threshold_percent: number;
+  idle_cash_threshold_amount: string;
+  idle_cash_threshold_days: number;
 }