fix(backend): spec-bug fixes for T10 — Match.claim_id non-unique + reconcile test dates

Three spec-bug fixes from T10 implementation:

- Match.claim_id: drop UNIQUE constraint (T4 schema error) — reversals add a 2nd row per claim (audit trail). Add explicit non-unique index ix_matches_claim_id to preserve query performance. Mirrored in ORM and migration 0001_initial.sql.
- test_run_orphan_remit_leaves_claim_unmatched: claim PCN-A does not match remit PCN-NEW, so unmatched_claims must be 1, not 0.
- test_run_reversal_flips_paid_to_reversed: claim service_date_from was 9 days before reversal remit service_date (outside default 7-day window) so no match occurred; changed to 5 days apart so match() picks the claim.
- test_match_unique_per_claim: in test_db_models.py asserted the now-removed UNIQUE behavior; renamed and inverted to assert the audit-trail design (two Match rows per claim allowed).

backend/tests/test_reconcile.py

@@ -290,13 +290,18 @@ def test_run_orphan_remit_leaves_claim_unmatched(fixture_835):
 
     assert result.matched == 0
     assert result.unmatched_remittances == 1
-    assert result.unmatched_claims == 0
+    # Spec-bug fix (T10): claim PCN-A does not match remit PCN-NEW, so the
+    # claim stays in the unmatched bucket. Original spec asserted 0.
+    assert result.unmatched_claims == 1
 
 
 def test_run_reversal_flips_paid_to_reversed(fixture_835):
     with db.SessionLocal()() as s:
         _make_batch(s)
-        _make_claim(s, "CLM-1", "PCN-A", "100.00", date(2026, 6, 1),
+        # Spec-bug fix (T10): dates are 5 days apart so match() picks this claim
+        # (default window_days=7). Original spec used 2026-06-01 vs 2026-06-10
+        # (9 days, outside window) which produced no match.
+        _make_claim(s, "CLM-1", "PCN-A", "100.00", date(2026, 6, 5),
                     state=ClaimState.PAID)
         # Match row already exists from prior reconcile.
         s.add(Match(