feat(db): migration 0014 relaxes claims/remittances PK to (batch_id, id)

Migration 0014 changes the PRIMARY KEYs of claims and remittances from
single-column id to composite (batch_id, id). Enables the spec'd
cross-batch CLM01 / CLP01 collision workflow: same CLM01 in multiple
batches is now representable (resubmits), pre-flight dedup 409 path is
genuinely exercisable, force-insert can skip pre-existing duplicates.

Strategy: PRAGMA defer_foreign_keys = ON + table recreation for every
table whose FKs pointed at the old single-column PK. Tables recreated:
remittances, claims, matches, cas_adjustments, service_line_payments,
line_reconciliations. Each child table gains a batch_id (or
remittance_batch_id) column for the composite FK side; INSERT INTO new
SELECT FROM old JOINs populate it from the already-recreated parent.

Cross-table FKs (remittances.claim_id, claims.matched_remittance_id)
cannot be SQL-enforced with composite PKs (SQLite has no ALTER
CONSTRAINT). Dropped at SQL level; enforced via application-layer
invariants in store.manual_match / manual_unmatch / reconcile.run and
the dedup.preflight_* helpers.

ORM updates (db.py):
- Claim / Remittance: composite PK via explicit PrimaryKeyConstraint in
  __table_args__ (column order matches SQL: batch_id, id).
- New Claim.matched_remittance_batch_id column.
- Match / CasAdjustment / ServiceLinePayment / LineReconciliation:
  added the batch side of their composite FK to the parent table.
- SQLAlchemy before_insert events auto-populate the batch side of the
  composite FK from session.new, then identity_map, then SQL fallback.

Production code updates:
- store.manual_match / manual_unmatch: also write
  matched_remittance_batch_id on the claim (was missing).
- api.py manual-match endpoint: same fix.
- reconcile.run: same fix for auto-matched pairs.

Test updates: replaced s.get(Claim, X) with the composite key
(batch_id, id) where batch_id is known, or s.query().filter().first()
where the test only knows the id. Tests that previously inserted a Match
row pointing at a non-existent Remittance now seed the parent Remittance
so the new NOT NULL composite FK is satisfied.

backend/tests/test_store_reconcile.py

@@ -221,7 +221,8 @@ def test_manual_match_pairs_claim_with_orphan_remit():
     # Match row was persisted with strategy="manual".
     with db.SessionLocal()() as session:
         from cyclone.db import Claim, Match
-        claim_row = session.get(Claim, claim_id)
+        # Migration 0014: composite PK — claim ingested under batch "b-837".
+        claim_row = session.get(Claim, ("b-837", claim_id))
         assert claim_row is not None
         assert claim_row.matched_remittance_id == remit_id
         match_rows = (
@@ -417,7 +418,8 @@ def test_manual_match_populates_line_reconciliation_rows():
         assert unmatched.service_line_payment_id is None
 
         # Remittance aggregates were recomputed (0 CAS in this test).
-        r = session.get(Remittance, remit_id)
+        # Migration 0014: composite PK — remit ingested under batch "b-835-sp7".
+        r = session.get(Remittance, ("b-835-sp7", remit_id))
         assert r is not None
         assert r.adjustment_amount == Decimal("0")
         assert r.claim_level_adjustment_amount == Decimal("0")
@@ -583,7 +585,8 @@ def test_manual_match_idempotent_line_reconciliation():
 
         # CLP-level CAS aggregate now reflects the inserted row.
         from cyclone.db import Remittance
-        r = session.get(Remittance, remit_id)
+        # Migration 0014: composite PK — remit ingested under batch "b-835-idemp".
+        r = session.get(Remittance, ("b-835-idemp", remit_id))
         assert r is not None
         assert r.adjustment_amount == Decimal("20.00")
         assert r.claim_level_adjustment_amount == Decimal("20.00")