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_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()