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/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):