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/api/v1/endpoints/transactions.py

@@ -93,7 +93,7 @@ def list_transactions(
             Transaction.date >= datetime.fromisoformat(start_date),
             Transaction.date < datetime.fromisoformat(end_date),
         )
-    query = query.order_by(col(Transaction.date).desc()).offset(offset).limit(limit)
+    query = query.order_by(col(Transaction.date).desc(), col(Transaction.id).desc()).offset(offset).limit(limit)
     return session.exec(query).all()
 
 
@@ -166,7 +166,7 @@ def recent_transactions(
     query = (
         select(Transaction)
         .where(Transaction.source == TransactionSource.CREDIT_CARD)
-        .order_by(col(Transaction.date).desc())
+        .order_by(col(Transaction.date).desc(), col(Transaction.id).desc())
         .limit(limit)
     )
     return session.exec(query).all()