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/alembic/versions/7884505b16b3_fk_delete_rules.py

@@ -0,0 +1,61 @@
+"""fk delete rules
+
+Revision ID: 7884505b16b3
+Revises: 7f567c8bafdb
+Create Date: 2026-06-10 13:50:39.153585
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+import sqlmodel
+
+
+# revision identifiers, used by Alembic.
+revision: str = '7884505b16b3'
+down_revision: Union[str, Sequence[str], None] = '7f567c8bafdb'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Add delete rules to the three FKs (review ARCH-09/BE-11).
+
+    Transactions and recurring items outlive their category (SET NULL);
+    water readings die with their receipt (CASCADE). Constraint names match
+    both pre-Alembic databases and fresh baseline builds (Postgres default
+    naming).
+    """
+    op.drop_constraint("transaction_category_id_fkey", "transaction", type_="foreignkey")
+    op.create_foreign_key(
+        "transaction_category_id_fkey", "transaction", "category",
+        ["category_id"], ["id"], ondelete="SET NULL",
+    )
+    op.drop_constraint("recurringitem_category_id_fkey", "recurringitem", type_="foreignkey")
+    op.create_foreign_key(
+        "recurringitem_category_id_fkey", "recurringitem", "category",
+        ["category_id"], ["id"], ondelete="SET NULL",
+    )
+    op.drop_constraint("watermeterreading_receipt_id_fkey", "watermeterreading", type_="foreignkey")
+    op.create_foreign_key(
+        "watermeterreading_receipt_id_fkey", "watermeterreading", "municipalreceipt",
+        ["receipt_id"], ["id"], ondelete="CASCADE",
+    )
+
+def downgrade() -> None:
+    op.drop_constraint("watermeterreading_receipt_id_fkey", "watermeterreading", type_="foreignkey")
+    op.create_foreign_key(
+        "watermeterreading_receipt_id_fkey", "watermeterreading", "municipalreceipt",
+        ["receipt_id"], ["id"],
+    )
+    op.drop_constraint("recurringitem_category_id_fkey", "recurringitem", type_="foreignkey")
+    op.create_foreign_key(
+        "recurringitem_category_id_fkey", "recurringitem", "category",
+        ["category_id"], ["id"],
+    )
+    op.drop_constraint("transaction_category_id_fkey", "transaction", type_="foreignkey")
+    op.create_foreign_key(
+        "transaction_category_id_fkey", "transaction", "category",
+        ["category_id"], ["id"],
+    )