refactor: optimize and restructure ratings extraction logic for performance and maintainability

- Moved validation logic for rating rows to a dedicated `_validate_rating_row` function.
- Refactored `_process_rating_rows` to perform bulk loading using COPY for improved efficiency.
- Introduced staging table with deduplication and atomic swap to ensure data integrity during ratings reload.
- Added helper functions for staging table creation, deduplication, swap, and cleanup.

Author: ayuki-joto