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_acks.py

@@ -51,16 +51,16 @@ def test_migration_0002_creates_acks_table():
 
 def test_migration_latest_idempotent_on_fresh_db():
     """Re-running the migration on the same DB must be a no-op (PRAGMA
-    user_version already at the latest version — currently 3 after the
-    0003 constraint drop was added)."""
+    user_version already at the latest version — currently 4 after the
+    0004 rejection columns + state-history index was added by SP6)."""
     with db.engine().begin() as c:
         v1 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v1 == 3
+    assert v1 == 4
     # A second run should not raise and should not bump the version.
     db_migrate.run(db.engine())
     with db.engine().begin() as c:
         v2 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v2 == 3
+    assert v2 == 4
 
 
 def test_add_ack_persists_row():