When an admin was deleted, the ON DELETE SET NULL foreign key would set
created_by_admin_id to NULL on all their libraries. This caused the scanner
to fail to find an admin ID for broadcasting scan-complete WebSocket messages.
- On admin deletion, reassign all libraries and media items to the next admin
- Prevents created_by_admin_id from ever being NULL on active libraries
- Uses new ReassignLibraries and ReassignMediaItems DB queries