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_state.py

@@ -111,7 +111,9 @@ def test_rejection_moves_submitted_claim_to_rejected_state():
     assert result.matched == ["CLP-1"]
     assert result.orphans == []
     with db.SessionLocal()() as s:
-        c = s.get(Claim, "CLP-1")
+        # Migration 0014: composite PK — claim is in batch "B-1"; use
+        # filter-by-id since the test has only one claim with this id.
+        c = s.query(Claim).filter(Claim.id == "CLP-1").first()
         assert c.state == ClaimState.REJECTED
         assert c.rejected_at is not None
         assert "999" in (c.rejection_reason or "")
@@ -130,7 +132,8 @@ def test_accepted_set_leaves_claim_alone():
 
     assert result.matched == []
     with db.SessionLocal()() as s:
-        c = s.get(Claim, "CLP-1")
+        # Migration 0014: composite PK — see test_rejection_moves...
+        c = s.query(Claim).filter(Claim.id == "CLP-1").first()
         assert c.state == ClaimState.SUBMITTED
         assert c.rejected_at is None
         assert c.rejection_reason is None
@@ -162,5 +165,6 @@ def test_already_rejected_claim_is_idempotent():
 
     assert result.matched == []   # no-op
     with db.SessionLocal()() as s:
-        c = s.get(Claim, "CLP-1")
+        # Migration 0014: composite PK — see test_rejection_moves...
+        c = s.query(Claim).filter(Claim.id == "CLP-1").first()
         assert c.state == ClaimState.REJECTED