test(sp39): bump PRAGMA user_version assertion from 20 to 21

Migration 0021_resubmissions.sql (SP39 Task 2) advances the head from
20 to 21. Both test_migration_0020.migrated_engine and
test_acks.test_migration_latest_idempotent_on_fresh_db pinned the old
head; update both. No production behavior change.

backend/tests/test_acks.py

@@ -51,7 +51,7 @@ 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 20 after
+    user_version already at the latest version — currently 21 after
     0004-0006 line_reconciliation, 0005 ta1_acks, SP9's 0007
     providers/payers/clearhouse, SP10's 0008 payer_rejected,
     SP11's 0009 audit_log, SP14's 0010 payer_rejected_acknowledged,
@@ -61,16 +61,17 @@ def test_migration_latest_idempotent_on_fresh_db():
     claims.matched_remittance_id index, SP27-Task 17's 0017
     claim.patient_control_number backfill UPDATE, SP28's 0018
     claim_acks join table, SP32's 0019
-    rendering_provider_npi + service_provider_npi, and SP37's 0020
-    transaction_set_control_number)."""
+    rendering_provider_npi + service_provider_npi, SP37's 0020
+    transaction_set_control_number, and SP39's 0021 resubmissions
+    audit table)."""
     with db.engine().begin() as c:
         v1 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v1 == 20
+    assert v1 == 21
     # 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 == 20
+    assert v2 == 21
 
 
 def test_add_ack_persists_row():

backend/tests/test_migration_0020.py

@@ -102,10 +102,10 @@ def migrated_engine(tmp_path: Path, monkeypatch: pytest.MonkeyPatch):
     engine = _fresh_engine(tmp_path / "mig0020.db")
     db_migrate.run(engine)
 
-    # Confirm head is 20 (every migration applied).
+    # Confirm head is 21 (every migration applied, including SP39's 0021).
     with engine.connect() as conn:
         v = conn.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v == 20, f"expected migration head=20, got {v}"
+    assert v == 21, f"expected migration head=21, got {v}"
 
     yield engine
     engine.dispose()