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_api_277ca.py

@@ -98,8 +98,10 @@ class TestParse277CAEndpointHappyPath:
             files={"file": ("minimal_277ca.txt", text, "text/plain")},
         )
         with db.SessionLocal()() as s:
-            c1 = s.get(Claim, "c1")
-            c2 = s.get(Claim, "c2")
+            # Migration 0014: composite PK (batch_id, id). The _seed_claim
+            # helper uses batch_id="BATCH-1".
+            c1 = s.get(Claim, ("BATCH-1", "c1"))
+            c2 = s.get(Claim, ("BATCH-1", "c2"))
         assert c1.payer_rejected_at is None
         assert c2.payer_rejected_at is not None
         assert c2.payer_rejected_status_code == "A6"