fix: restrict transaction splits to one parent category, show subcategory breakdown

Раньше сплит позволял смешивать вообще любые категории. Реальная задача —
разложить одну покупку по подкатегориям ОДНОЙ базовой категории (100 € на
продукты: сколько на сладости, сколько на уход). А раз Dashboard и рейтинг
категорий сворачивают подкатегории в родителя, эта разбивка нигде не была
видна.

- _build_splits() требует, чтобы все категории сплита были одной родительской
  категорией или её прямыми подкатегориями.
- category_rollup.py отдаёт для каждого родительского тотала список
  category-источников, которые в него скинулись (children) — пусто, если
  источник один.
- Dashboard и рейтинг категорий: строка с непустым children раскрывается по
  клику, показывая вклад каждой подкатегории.
- Форма транзакции: выбор категории при сплите — сначала базовая (верхний
  уровень), затем строки сплита выбирают саму базовую или её подкатегорию.

backend/app/api/routes/transactions.py

@@ -52,13 +52,15 @@ _TYPE_TO_CATEGORY_KIND = {
 
 async def _ensure_category_matches_type(
     session: AsyncSession, category_id: int | None, transaction_type: TransactionType
-) -> None:
+) -> Category | None:
     """A category picked for an income transaction must itself be an income
     category (and likewise for expense) — otherwise the dashboard's spending
     breakdown, which only joins EXPENSE-typed rows, would silently misclassify
-    the entry."""
+    the entry. Returns the fetched category (or None for category_id=None) so
+    callers that also need the row itself — _build_splits, below — don't have
+    to fetch it a second time."""
     if category_id is None:
-        return
+        return None
     expected_kind = _TYPE_TO_CATEGORY_KIND.get(transaction_type)
     category = await session.get(Category, category_id)
     if category is None:
@@ -68,13 +70,32 @@ async def _ensure_category_matches_type(
             status_code=400,
             detail=f"Category '{category.name}' is a {category.kind.value} category and cannot be used for a {transaction_type.value} transaction",
         )
+    return category
 
 
 async def _build_splits(
     session: AsyncSession, splits: list[TransactionSplitInput], transaction_type: TransactionType
 ) -> list[TransactionSplit]:
+    """A split's whole point is dividing one purchase's total across the
+    *subcategories of one parent* (a hypermarket receipt: part groceries ->
+    Sweets, part -> Alcohol) — not across unrelated top-level categories, or
+    the numbers would roll up into two different parents and the "spent X on
+    Groceries, split between Sweets/Alcohol" picture the feature exists for
+    falls apart. Each split may point at that parent category itself (an
+    unspecified-subcategory line) or at any one of its direct children —
+    enforced by requiring every split's own top-level ancestor
+    (parent_id, or its own id if it has none) to agree.
+    """
+    top_level_ids: set[int] = set()
     for split in splits:
-        await _ensure_category_matches_type(session, split.category_id, transaction_type)
+        category = await _ensure_category_matches_type(session, split.category_id, transaction_type)
+        assert category is not None  # split.category_id is required (not Optional) on the schema
+        top_level_ids.add(category.parent_id if category.parent_id is not None else category.id)
+    if len(top_level_ids) > 1:
+        raise HTTPException(
+            status_code=400,
+            detail="All split categories must be the same parent category or its direct subcategories",
+        )
     return [TransactionSplit(category_id=s.category_id, amount=s.amount, note=s.note) for s in splits]
 
 
@@ -196,17 +217,13 @@ async def bulk_create_transactions(
     instead of leaving a half-imported statement behind."""
     for item in payload.items:
         await _ensure_category_matches_type(session, item.category_id, item.type)
-        for split in item.splits or []:
-            await _ensure_category_matches_type(session, split.category_id, item.type)
 
     transactions = []
     for item in payload.items:
         transaction = Transaction(**item.model_dump(exclude={"tag_ids", "splits"}))
         transaction.tags = await _resolve_tags(session, item.tag_ids)
         if item.splits:
-            transaction.splits = [
-                TransactionSplit(category_id=s.category_id, amount=s.amount, note=s.note) for s in item.splits
-            ]
+            transaction.splits = await _build_splits(session, item.splits, item.type)
         transactions.append(transaction)
 
     session.add_all(transactions)

backend/app/schemas/dashboard.py

@@ -1,6 +1,18 @@
 from decimal import Decimal
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
+
+
+class CategoryBreakdownChildItem(BaseModel):
+    """One subcategory's (or the parent's own direct, un-subcategorized)
+    share of a CategoryBreakdownItem's total — see
+    services/category_rollup.py's CategoryRollupChildItem."""
+
+    category_id: int
+    name: str
+    color: str
+    icon: str | None
+    amount: Decimal
 
 
 class CategoryBreakdownItem(BaseModel):
@@ -10,6 +22,10 @@ class CategoryBreakdownItem(BaseModel):
     icon: str | None
     amount: Decimal
     percent: float
+    # Populated only when this slice's spend came from more than one
+    # distinct category (subcategories, or a mix of the parent itself and
+    # its children) — e.g. a receipt split across "Groceries" subcategories.
+    children: list[CategoryBreakdownChildItem] = Field(default_factory=list)
 
 
 class DashboardSummary(BaseModel):

backend/app/schemas/reports.py

@@ -1,7 +1,19 @@
 from datetime import date as date_
 from decimal import Decimal
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
+
+
+class CategoryRankingChildItem(BaseModel):
+    """One subcategory's (or the parent's own direct, un-subcategorized)
+    share of a CategoryRankingItem's total — see
+    services/category_rollup.py's CategoryRollupChildItem."""
+
+    category_id: int
+    name: str
+    color: str
+    icon: str | None
+    amount: Decimal
 
 
 class CategorySpendingPoint(BaseModel):
@@ -31,6 +43,10 @@ class CategoryRankingItem(BaseModel):
     amount: Decimal
     percent: float
     transaction_count: int
+    # Populated only when this category's spend came from more than one
+    # distinct category (subcategories, or a mix of itself and its
+    # children) — e.g. a receipt split across "Groceries" subcategories.
+    children: list[CategoryRankingChildItem] = Field(default_factory=list)
 
 
 class CategoryRankingReport(BaseModel):

backend/app/services/category_rollup.py

@@ -13,7 +13,7 @@ funded partly by plain transactions and partly by split lines still shows
 one correct total.
 """
 from collections import defaultdict
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from datetime import date as date_
 from decimal import Decimal
 
@@ -25,6 +25,20 @@ from app.models.enums import TransactionType
 from app.models.transaction import Transaction, TransactionSplit
 
 
+@dataclass
+class CategoryRollupChildItem:
+    """One leaf category's share of its parent's total — a real
+    subcategory, or the parent category itself when some of the spend was
+    filed directly under it with no more specific subcategory (category_id
+    equals the enclosing CategoryRollupItem.category_id in that case)."""
+
+    category_id: int
+    name: str
+    color: str
+    icon: str | None
+    amount: Decimal
+
+
 @dataclass
 class CategoryRollupItem:
     category_id: int
@@ -37,6 +51,13 @@ class CategoryRollupItem:
     # transaction counts once per top-level category it touches, same as a
     # plain one counts once for its own category.
     transaction_count: int
+    # Populated only when more than one distinct category actually fed this
+    # total (a subcategory, or a direct-to-parent line, or several of
+    # either) — e.g. a receipt split between "Sweets" and "Alcohol" under
+    # "Groceries". A single-source category (the common case, no splitting
+    # or subcategories involved) leaves this empty rather than showing a
+    # redundant one-item breakdown of itself.
+    children: list[CategoryRollupChildItem] = field(default_factory=list)
 
 
 async def _raw_category_contributions(
@@ -92,15 +113,34 @@ async def rollup_spending_by_top_level_category(
 
     amount_by_effective: dict[int, Decimal] = defaultdict(Decimal)
     txn_ids_by_effective: dict[int, set[int]] = defaultdict(set)
+    # effective (top-level) category id -> {actual leaf category_id: amount}
+    # — the leaf is the same as the effective id when a contribution was
+    # filed directly on the parent, and a genuine child id otherwise.
+    amount_by_leaf: dict[int, dict[int, Decimal]] = defaultdict(lambda: defaultdict(Decimal))
     for transaction_id, category_id, amount in contributions:
         category = categories_by_id.get(category_id)
         effective_id = category.parent_id if category and category.parent_id is not None else category_id
         amount_by_effective[effective_id] += amount
+        amount_by_leaf[effective_id][category_id] += amount
         txn_ids_by_effective[effective_id].add(transaction_id)
 
     items: list[CategoryRollupItem] = []
     for effective_id, amount in amount_by_effective.items():
         category = categories_by_id.get(effective_id)
+        leaf_amounts = amount_by_leaf[effective_id]
+        children: list[CategoryRollupChildItem] = []
+        if len(leaf_amounts) > 1:
+            for leaf_id, leaf_amount in sorted(leaf_amounts.items(), key=lambda pair: -pair[1]):
+                leaf_category = categories_by_id.get(leaf_id)
+                children.append(
+                    CategoryRollupChildItem(
+                        category_id=leaf_id,
+                        name=leaf_category.name if leaf_category else "?",
+                        color=leaf_category.color if leaf_category else "#898781",
+                        icon=leaf_category.icon if leaf_category else None,
+                        amount=leaf_amount,
+                    )
+                )
         items.append(
             CategoryRollupItem(
                 category_id=effective_id,
@@ -110,6 +150,7 @@ async def rollup_spending_by_top_level_category(
                 sort_order=category.sort_order if category else 0,
                 amount=amount,
                 transaction_count=len(txn_ids_by_effective[effective_id]),
+                children=children,
             )
         )
     items.sort(key=lambda item: (-item.amount, item.sort_order))

backend/app/services/dashboard_service.py

@@ -8,7 +8,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.models.enums import TransactionType
 from app.models.transaction import Transaction
-from app.schemas.dashboard import CategoryBreakdownItem, DashboardSummary
+from app.schemas.dashboard import CategoryBreakdownChildItem, CategoryBreakdownItem, DashboardSummary
 from app.services.category_rollup import rollup_spending_by_top_level_category
 
 # Categorical slots are capped at 8 (dataviz skill: a 9th series folds into "Other",
@@ -54,6 +54,13 @@ async def get_dashboard_summary(session: AsyncSession, year: int, month: int) ->
         CategoryBreakdownItem(
             category_id=row.category_id, name=row.name, color=row.color, icon=row.icon,
             amount=row.amount, percent=_percent(row.amount),
+            children=[
+                CategoryBreakdownChildItem(
+                    category_id=child.category_id, name=child.name, color=child.color, icon=child.icon,
+                    amount=child.amount,
+                )
+                for child in row.children
+            ],
         )
         for row in top_rows
     ]

backend/app/services/reports_service.py

@@ -16,6 +16,7 @@ from app.models.category import Category
 from app.models.enums import CategoryKind, TransactionType
 from app.models.transaction import Transaction, TransactionSplit
 from app.schemas.reports import (
+    CategoryRankingChildItem,
     CategoryRankingItem,
     CategoryRankingReport,
     CategorySpendingPoint,
@@ -155,6 +156,13 @@ async def get_category_ranking_report(
             amount=row.amount,
             percent=_percent(row.amount),
             transaction_count=row.transaction_count,
+            children=[
+                CategoryRankingChildItem(
+                    category_id=child.category_id, name=child.name, color=child.color, icon=child.icon,
+                    amount=child.amount,
+                )
+                for child in row.children
+            ],
         )
         for row in rows
     ]