feat(sp6): add REJECTED to ClaimState enum

T1 of the SP6 workflow automation plan. Paired with the existing test
in test_db_models.py that asserts 8 ClaimState values.

backend/src/cyclone/db.py

@@ -123,6 +123,7 @@ def SessionLocal() -> sessionmaker[sa.orm.Session]:
 class ClaimState(str, enum.Enum):
     SUBMITTED = "submitted"
     RECEIVED = "received"
+    REJECTED = "rejected"     # NEW — set by 999 AK9 set-level R/E
     PAID = "paid"
     PARTIAL = "partial"
     DENIED = "denied"