fix(budgets): count subcategory spending against the parent's budget (#6)

get_budget_status() summed only transactions whose category_id matched the
budgeted category exactly, while the Dashboard breakdown and the category
report both roll a subcategory's spending up into its parent
(coalesce(parent_id, id) in dashboard_service.py and reports_service.py).

With one level of subcategories — the app supports exactly one — a budget on
"Housing & Utilities" ignored everything filed under "Rent": the Dashboard
showed 900 spent for the month and the Budget page showed 0 of 1000 for the
same category, same month. Since insights_service's over-budget alert reads
this same status, the alert stayed silent too.

Budgets now count the budgeted category plus its children, matching the
rollup the other two screens already use. A subcategory that has a budget of
its own keeps tracking its own spending, so both bars move against their own
limits — that case is pinned by a test as well.

backend/app/services/budget_service.py

@@ -3,6 +3,7 @@ get_budget_status(), which compares each limit against actual spend for a
 given month — the data behind the Budget page's progress bars and the
 budget_exceeded proactive alert (services/insights_service.py)."""
 import calendar
+from collections import defaultdict
 from datetime import date
 from decimal import Decimal
 
@@ -73,10 +74,28 @@ async def get_budget_status(session: AsyncSession, year: int, month: int) -> Bud
     if not budgets:
         return BudgetStatusResponse(year=year, month=month, items=[])
 
+    # A budget on a top-level category covers its subcategories too — the
+    # same rollup the Dashboard breakdown and the category report already do
+    # (coalesce(parent_id, id) in dashboard_service/reports_service).
+    # Counting only exact category_id matches meant a month could read as 900
+    # spent on the Dashboard and 0 against its own budget. A subcategory that
+    # has a budget of its own still tracks its own spending: both bars move,
+    # each against its own limit.
+    budget_category_ids = [b.category_id for b in budgets]
+    children_by_parent: dict[int, list[int]] = defaultdict(list)
+    child_rows = await session.execute(
+        select(Category.id, Category.parent_id).where(Category.parent_id.in_(budget_category_ids))
+    )
+    for child_id, parent_id in child_rows.all():
+        children_by_parent[parent_id].append(child_id)
+
+    counted_ids = set(budget_category_ids).union(
+        child_id for children in children_by_parent.values() for child_id in children
+    )
     spent_stmt = (
         select(Transaction.category_id, func.coalesce(func.sum(Transaction.amount), 0))
         .where(
-            Transaction.category_id.in_([b.category_id for b in budgets]),
+            Transaction.category_id.in_(counted_ids),
             Transaction.type == TransactionType.EXPENSE,
             Transaction.date >= start,
             Transaction.date <= end,
@@ -87,7 +106,10 @@ async def get_budget_status(session: AsyncSession, year: int, month: int) -> Bud
 
     items = []
     for budget in budgets:
-        spent = spent_by_category.get(budget.category_id, Decimal("0"))
+        spent = spent_by_category.get(budget.category_id, Decimal("0")) + sum(
+            (spent_by_category.get(child_id, Decimal("0")) for child_id in children_by_parent[budget.category_id]),
+            Decimal("0"),
+        )
         percent = float(spent / budget.monthly_limit * 100) if budget.monthly_limit else 0.0
         items.append(
             BudgetStatus(

backend/tests/test_budgets.py

@@ -80,3 +80,55 @@ async def test_delete_budget_removes_it_from_status(client: AsyncClient, account
 
     status = await client.get("/budgets/status", params={"year": 2026, "month": 8})
     assert status.json()["items"] == []
+
+
+async def _subcategory(client: AsyncClient, parent_id: int, name: str = "Rent") -> int:
+    resp = await client.post(
+        "/categories",
+        json={"name": name, "kind": "expense", "color": "#7a869a", "parent_id": parent_id},
+    )
+    return resp.json()["id"]
+
+
+async def test_budget_on_a_parent_category_counts_spending_in_its_subcategories(
+    client: AsyncClient, account_id, categories
+):
+    """The Dashboard breakdown and the category report both roll a
+    subcategory's spending up into its parent (coalesce(parent_id, id)). A
+    budget set on the parent has to agree with them, or the same month reads
+    as 900 spent on one screen and 0 on the other."""
+    parent_id = categories["Housing & Utilities"]["id"]
+    child_id = await _subcategory(client, parent_id)
+    await client.post("/budgets", json={"category_id": parent_id, "monthly_limit": "1000.00"})
+    await client.post(
+        "/transactions", json=txn_payload(account_id, category_id=child_id, amount="900.00", date="2026-03-10")
+    )
+
+    status = (await client.get("/budgets/status?year=2026&month=3")).json()["items"][0]
+    dashboard = (await client.get("/dashboard/summary?year=2026&month=3")).json()
+
+    assert money(status["spent"]) == money("900.00")
+    assert money(status["remaining"]) == money("100.00")
+    # The two screens must show the same number for the same category.
+    parent_slice = next(s for s in dashboard["spending_by_category"] if s["category_id"] == parent_id)
+    assert money(parent_slice["amount"]) == money(status["spent"])
+
+
+async def test_a_subcategory_keeps_its_own_budget_separate(client: AsyncClient, account_id, categories):
+    """Rolling children into a parent's budget must not swallow a budget set
+    on the child itself — both bars track the same spending, each against its
+    own limit."""
+    parent_id = categories["Housing & Utilities"]["id"]
+    child_id = await _subcategory(client, parent_id)
+    await client.post("/budgets", json={"category_id": parent_id, "monthly_limit": "1000.00"})
+    await client.post("/budgets", json={"category_id": child_id, "monthly_limit": "800.00"})
+    await client.post(
+        "/transactions", json=txn_payload(account_id, category_id=child_id, amount="900.00", date="2026-03-10")
+    )
+
+    items = {item["category_id"]: item for item in (await client.get("/budgets/status?year=2026&month=3")).json()["items"]}
+
+    assert money(items[child_id]["spent"]) == money("900.00")
+    assert items[child_id]["is_over_budget"] is True
+    assert money(items[parent_id]["spent"]) == money("900.00")
+    assert items[parent_id]["is_over_budget"] is False