Constraint and query pass: FK delete rules, grouped queries, tiebreakers

- FK rules via migration 7884505b16b3 (verified on dev with a rolled-
  back probe): transactions/recurring items SET NULL on category delete
  (this also fixes the 500 that DELETE /categories raised for in-use
  categories), water readings CASCADE with their receipt. Models declare
  ondelete to stay in sync. (ARCH-09, BE-11)
- compute_actuals_by_source: 3 grouped queries replace 10 single-
  aggregate round-trips; behavior pinned by the existing cycle suite.
  (BE-21)
- compute_cc_by_category prefetches category names (ARCH-15); agent
  pension latest-per-fund resolved in SQL (BE-04); municipal water
  consumption batched into one grouped query (BE-05).
- Deterministic pagination: date DESC, id DESC on all transaction
  listings. (ARCH-12)
- New agent-tool tests (session binding, JSON-safe output, batched
  queries): 64 tests green.

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

backend/app/models/models.py

@@ -157,7 +157,9 @@ class TransactionBase(SQLModel):
     source: TransactionSource = TransactionSource.CREDIT_CARD
     bank: Bank = Bank.BAC
     notes: Optional[str] = None
-    category_id: Optional[int] = Field(default=None, foreign_key="category.id")
+    category_id: Optional[int] = Field(
+        default=None, foreign_key="category.id", ondelete="SET NULL"
+    )
     deferred_to_next_cycle: bool = Field(
         default=False, sa_column_kwargs={"server_default": "false"}
     )
@@ -273,7 +275,9 @@ class RecurringItemBase(SQLModel):
         default=None,
         sa_column=Column(JSON, nullable=True),
     )
-    category_id: Optional[int] = Field(default=None, foreign_key="category.id")
+    category_id: Optional[int] = Field(
+        default=None, foreign_key="category.id", ondelete="SET NULL"
+    )
     is_active: bool = True
     notes: Optional[str] = None
 
@@ -470,7 +474,9 @@ class WaterMeterReadingBase(SQLModel):
     alcant_sanitario: Money = Field(default=Decimal("0"), max_digits=15, decimal_places=2)
     iva: Money = Field(default=Decimal("0"), max_digits=15, decimal_places=2)
     is_historical: bool = False
-    receipt_id: Optional[int] = Field(default=None, foreign_key="municipalreceipt.id")
+    receipt_id: Optional[int] = Field(
+        default=None, foreign_key="municipalreceipt.id", ondelete="CASCADE"
+    )
 
 
 class WaterMeterReading(WaterMeterReadingBase, table=True):