DuckDBService class

The DuckDBService class is defined within backend/app/services/duckdb_service.py. DuckDBService class uses the project_4.duckdb file to store and query project-specific data. PgDataService replaces DuckDBService for data storage and querying of extracted data. The new pg_data_service.py file provides PostgreSQL data management functionality replacing DuckDBService. The DATA_BACKEND feature flag can toggle between using PostgreSQL (PgDataService) and DuckDBService as the data backend. PgDataService replaced DuckDBService in extract.py and question_router.py to switch backend from DuckDB to PostgreSQL. The execute_query method is part of the DuckDB Service.