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/agent/tools.py

@@ -266,16 +266,27 @@ def get_pension_snapshots(
 ) -> list[dict]:
     """Pension fund snapshots. Each snapshot covers a period with balances,
     contributions, returns, fees and the ending balance (saldo_final)."""
-    q = select(PensionSnapshot).order_by(col(PensionSnapshot.period_end).desc())
+    if latest_only:
+        # Latest snapshot per fund resolved in SQL instead of scanning all rows
+        latest = (
+            select(
+                PensionSnapshot.fund.label("fund"),
+                func.max(PensionSnapshot.period_end).label("period_end"),
+            )
+            .group_by(PensionSnapshot.fund)
+            .subquery()
+        )
+        q = select(PensionSnapshot).join(
+            latest,
+            (PensionSnapshot.fund == latest.c.fund)
+            & (PensionSnapshot.period_end == latest.c.period_end),
+        )
+    else:
+        q = select(PensionSnapshot)
+    q = q.order_by(col(PensionSnapshot.period_end).desc())
     if fund:
         q = q.where(PensionSnapshot.fund == fund)
     rows = _s().exec(q).all()
-    if latest_only:
-        seen: dict[str, PensionSnapshot] = {}
-        for r in rows:
-            if r.fund.value not in seen:
-                seen[r.fund.value] = r
-        rows = list(seen.values())
     return [
         {
             "fund": r.fund.value,
@@ -322,11 +333,21 @@ def get_municipal_receipts(
         q = q.where(MunicipalReceipt.account == account)
     q = q.limit(limit)
     rows = _s().exec(q).all()
+    # One grouped query for all receipts instead of one per receipt
+    consumption: dict[int, float] = {}
+    ids = [r.id for r in rows if r.id is not None]
+    if ids:
+        grouped = _s().exec(
+            select(
+                WaterMeterReading.receipt_id,
+                func.sum(WaterMeterReading.consumption_m3),
+            )
+            .where(col(WaterMeterReading.receipt_id).in_(ids))
+            .group_by(WaterMeterReading.receipt_id)
+        ).all()
+        consumption = {rid: float(total) for rid, total in grouped}
     out: list[dict] = []
     for r in rows:
-        readings = _s().exec(
-            select(WaterMeterReading).where(WaterMeterReading.receipt_id == r.id)
-        ).all()
         out.append(
             {
                 "id": r.id,
@@ -338,7 +359,7 @@ def get_municipal_receipts(
                 "interests": float(r.interests),
                 "iva": float(r.iva),
                 "total": float(r.total),
-                "water_consumption_m3": float(sum(w.consumption_m3 for w in readings)),
+                "water_consumption_m3": consumption.get(r.id, 0.0),
             }
         )
     return out