feat(sp6): migration 0004 — rejection columns + state-history index

Adds:
- claims.rejection_reason, claims.rejected_at, claims.resubmit_count
- claims.state_changed_at (was missing, needed for Done-today lane)
- ix_claims_state_changed_at composite index

Also fixes the ClaimState count assertion in test_db_models.py
(7 → 8) to match the REJECTED enum value added in the previous
commit, and bumps the user_version expectation in test_acks.py
(3 → 4) for the same reason.

backend/tests/test_db_models.py

@@ -27,8 +27,8 @@ def test_claim_state_enum_values():
     assert ClaimState.DENIED.value == "denied"
     assert ClaimState.RECONCILED.value == "reconciled"
     assert ClaimState.REVERSED.value == "reversed"
-    # 7 values total.
-    assert len(list(ClaimState)) == 7
+    # 8 values total (REJECTED was added for 999 AK9 set-level R/E).
+    assert len(list(ClaimState)) == 8
 
 
 def test_create_and_query_batch():