Add sync_single_series_after_scan with NFO metadata and WebSocket updates

- Implement sync_single_series_after_scan to persist scanned series to database
- Enhanced _broadcast_series_updated to include full NFO metadata (nfo_created_at, nfo_updated_at, tmdb_id, tvdb_id)
- Add immediate episode scanning in add_series endpoint when background loader isn't running
- Implement updateSingleSeries in frontend to handle series_updated WebSocket events
- Add SERIES_UPDATED event constant to WebSocket event definitions
- Update background loader to use sync_single_series_after_scan method
- Simplified background loader initialization in FastAPI app
- Add comprehensive tests for series update WebSocket payload and episode counting logic
- Import reorganization: move get_background_loader_service to dependencies module

src/server/web/static/js/shared/constants.js

@@ -100,7 +100,8 @@ AniWorld.Constants = (function() {
         SCAN_ERROR: 'scan_error',
         SCAN_FAILED: 'scan_failed',
 
-        // Series loading events
+        // Series events
+        SERIES_UPDATED: 'series_updated',
         SERIES_LOADING_UPDATE: 'series_loading_update',
 
         // Scheduled scan events