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_inbox_endpoints.py

@@ -75,7 +75,9 @@ def test_match_endpoint_links_remit_to_claim(client: TestClient):
     assert r.status_code == 200, r.text
 
     with db.SessionLocal()() as s:
-        c = s.get(Claim, "C1")
+        # Migration 0014: composite PK — filter-by-id returns the single
+        # claim with this id (no cross-batch duplicates in this fixture).
+        c = s.query(Claim).filter(Claim.id == "C1").first()
         assert c.matched_remittance_id == "R1"
 
 
@@ -119,7 +121,8 @@ def test_resubmit_moves_rejected_to_submitted_and_increments_count(client: TestC
     assert r.status_code == 200, r.text
 
     with db.SessionLocal()() as s:
-        c = s.get(Claim, "C1")
+        # Migration 0014: composite PK (batch_id, id).
+        c = s.get(Claim, ("B-1", "C1"))
         assert c.state == ClaimState.SUBMITTED
         assert c.rejection_reason is None
         assert c.resubmit_count == 3
@@ -175,7 +178,9 @@ def _seed_rejected_claim_with_raw_837(client: TestClient) -> str:
     real_id = r.json()["claims"][0]["claim_id"]
     _seed_batch()
     with db.SessionLocal()() as s:
-        c = s.get(Claim, real_id)
+        # Migration 0014: filter-by-id since the parse-837 batch id is
+        # a fresh UUID we don't know here.
+        c = s.query(Claim).filter(Claim.id == real_id).first()
         assert c is not None
         c.state = ClaimState.REJECTED
         c.rejection_reason = "test fixture"
@@ -216,7 +221,8 @@ def test_resubmit_with_download_returns_zip_with_one_x12_per_accepted_claim(clie
 
     # Side effect: claim actually moved to SUBMITTED.
     with db.SessionLocal()() as s:
-        c = s.get(Claim, real_id)
+        # Migration 0014: filter-by-id (batch_id is a fresh UUID).
+        c = s.query(Claim).filter(Claim.id == real_id).first()
         assert c.state == ClaimState.SUBMITTED