feat(sp27): pin Claim↔Remit matched-pair invariant + startup drift audit

Add check_matched_pair_drift() at module level in store.py — read-only
audit of the Claim.matched_remittance_id ↔ Remittance.claim_id FK pair.
Logs WARNING on drift with up to 5 examples of each of two cases
(claim-side and remit-side), returns the count of drifted rows. Wired
into api.py::lifespan right after db.init_db() and ensure_clearhouse_seeded(),
wrapped in try/except so a query failure logs but doesn't crash boot.

The symmetric-write + symmetric-clear invariants on
manual_match / manual_unmatch were already correct; this commit pins
them with focused regression tests in test_store_match_invariant.py
(8 tests covering both directions, both happy-path, rollback pin, and
drift-check unit behavior). PCN asymmetry in the fixture prevents the
auto-match inside CycloneStore.add (Task 10) from pre-pairing, so
manual_match is the only writer.

Migration 0016 adds the missing ix_claims_matched_remittance_id index —
the drift check scans WHERE matched_remittance_id IS NOT NULL on every
boot and would become a full-table scan past ~10k claims. Symmetric
with ix_remittances_claim_id (added in 0007). Migration tests bumped
from head=15 to head=16.

backend/tests/test_acks.py

@@ -51,21 +51,22 @@ def test_migration_0002_creates_acks_table():
 
 def test_migration_latest_idempotent_on_fresh_db():
     """Re-running the migration on the same DB must be a no-op (PRAGMA
-    user_version already at the latest version — currently 15 after
+    user_version already at the latest version — currently 16 after
     0004-0006 line_reconciliation, 0005 ta1_acks, SP9's 0007
     providers/payers/clearhouse, SP10's 0008 payer_rejected,
     SP11's 0009 audit_log, SP14's 0010 payer_rejected_acknowledged,
     SP16's 0011 processed_inbound_files, SP17's 0012 db_backups,
     SP-auth's 0013 users + sessions, SP-audit's 0014 audit_log.user_id,
-    SP22's 0015 drop_claims_unique_constraint)."""
+    SP22's 0015 drop_claims_unique_constraint, SP27-Task 11's 0016
+    claims.matched_remittance_id index)."""
     with db.engine().begin() as c:
         v1 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v1 == 15
+    assert v1 == 16
     # A second run should not raise and should not bump the version.
     db_migrate.run(db.engine())
     with db.engine().begin() as c:
         v2 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v2 == 15
+    assert v2 == 16
 
 
 def test_add_ack_persists_row():