- Set `search_path` for all database functions, as this is recommended
by the Supabase security advisor
- Fix `items_delete` functions: The function was never working, which
caused that we saved to many items for a source, which slowed down our
performance
- Revoke execution rights for users for the `sources_delete_files` and
`items_delete` functions
- Add E2E test, to check if users can call the database functions