M5: enable SQLite foreign_keys pragma for DB-level FK enforcement

app/db.py

@@ -28,6 +28,7 @@ def _get_engine(database_url: str) -> Engine:
         def _enable_sqlite_wal(dbapi_connection, _connection_record):
             cursor = dbapi_connection.cursor()
             cursor.execute("PRAGMA journal_mode=WAL")
+            cursor.execute("PRAGMA foreign_keys=ON")
             cursor.close()
 
     return engine