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/src/cyclone/db.py

@@ -287,9 +287,12 @@ class Match(Base):
     __tablename__ = "matches"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    # Spec-bug fix (T10): not unique — a reversal adds a 2nd Match row
+    # for the same claim (original payment + reversal). The design intent
+    # is an audit trail.
     claim_id: Mapped[str] = mapped_column(
         String(64), ForeignKey("claims.id", ondelete="CASCADE"),
-        nullable=False, unique=True,
+        nullable=False,
     )
     remittance_id: Mapped[str] = mapped_column(
         String(64), ForeignKey("remittances.id", ondelete="CASCADE"), nullable=False
@@ -302,6 +305,7 @@ class Match(Base):
     is_reversal: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
 
     __table_args__ = (
+        Index("ix_matches_claim_id", "claim_id"),  # non-unique: reversals add a 2nd row
         Index("ix_matches_remittance_id", "remittance_id"),
         Index("ix_matches_matched_at", "matched_at"),
     )

backend/src/cyclone/migrations/0001_initial.sql

@@ -64,13 +64,16 @@ CREATE INDEX ix_cas_adjustments_remittance_id ON cas_adjustments(remittance_id);
 
 CREATE TABLE matches (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
-    claim_id TEXT NOT NULL UNIQUE REFERENCES claims(id) ON DELETE CASCADE,
+    -- Spec-bug fix (T10): not UNIQUE — a reversal adds a 2nd Match row for
+    -- the same claim (original payment + reversal). Audit trail design.
+    claim_id TEXT NOT NULL REFERENCES claims(id) ON DELETE CASCADE,
     remittance_id TEXT NOT NULL REFERENCES remittances(id) ON DELETE CASCADE,
     strategy TEXT NOT NULL,
     matched_at DATETIME NOT NULL,
     prior_claim_state TEXT,
     is_reversal INTEGER NOT NULL DEFAULT 0
 );
+CREATE INDEX ix_matches_claim_id ON matches(claim_id);
 CREATE INDEX ix_matches_remittance_id ON matches(remittance_id);
 CREATE INDEX ix_matches_matched_at ON matches(matched_at);
 

backend/tests/test_db_models.py

@@ -336,9 +336,12 @@ def test_create_match_and_activity_event():
         assert events[0].payload_json == {"matched": 1}
 
 
-def test_match_unique_per_claim():
-    """A Claim can only have one current Match row."""
-    import sqlalchemy.exc
+def test_match_multiple_rows_per_claim():
+    """Spec-bug fix (T10): a Claim can have multiple Match rows
+    (original payment + reversal = audit trail). UNIQUE on claim_id
+    was removed; the constraint test that previously lived here
+    asserted the old, incorrect design.
+    """
     with db.SessionLocal()() as s:
         s.add(Batch(
             id="b1", kind="835", input_filename="x",
@@ -352,10 +355,12 @@ def test_match_unique_per_claim():
                          status_code="1", received_at=datetime.now(timezone.utc)))
         s.add(Match(claim_id="CLM-1", remittance_id="CLP-1", strategy="auto",
                     matched_at=datetime.now(timezone.utc)))
+        s.add(Match(claim_id="CLM-1", remittance_id="CLP-2", strategy="manual",
+                    matched_at=datetime.now(timezone.utc),
+                    is_reversal=True, prior_claim_state=ClaimState.PAID))
+        # No IntegrityError — two Match rows per claim are now allowed.
         s.commit()
 
-        s.add(Match(claim_id="CLM-1", remittance_id="CLP-2", strategy="manual",
-                    matched_at=datetime.now(timezone.utc)))
-        with pytest.raises(sqlalchemy.exc.IntegrityError):
-            s.commit()
-        s.rollback()
+        rows = s.query(Match).filter(Match.claim_id == "CLM-1").all()
+        assert len(rows) == 2
+        assert {r.remittance_id for r in rows} == {"CLP-1", "CLP-2"}

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(