Fix genre whitelist settings not saving — add to allowed config keys

The genre_whitelist key was missing from the settings POST handler's
service whitelist, so the config was silently dropped on save.

web_server.py

@@ -5931,7 +5931,7 @@ def handle_settings():
             if 'active_media_server' in new_settings:
                 config_manager.set_active_media_server(new_settings['active_media_server'])
 
-            for service in ['spotify', 'plex', 'jellyfin', 'navidrome', 'soulseek', 'download_source', 'settings', 'database', 'metadata_enhancement', 'file_organization', 'playlist_sync', 'tidal', 'tidal_download', 'qobuz', 'hifi_download', 'deezer_download', 'lidarr_download', 'listenbrainz', 'acoustid', 'lastfm', 'genius', 'import', 'lossy_copy', 'listening_stats', 'ui_appearance', 'youtube', 'content_filter', 'itunes', 'm3u_export', 'musicbrainz', 'deezer', 'audiodb', 'metadata', 'hydrabase', 'security', 'discogs', 'library', 'discover', 'wishlist']:
+            for service in ['spotify', 'plex', 'jellyfin', 'navidrome', 'soulseek', 'download_source', 'settings', 'database', 'metadata_enhancement', 'file_organization', 'playlist_sync', 'tidal', 'tidal_download', 'qobuz', 'hifi_download', 'deezer_download', 'lidarr_download', 'listenbrainz', 'acoustid', 'lastfm', 'genius', 'import', 'lossy_copy', 'listening_stats', 'ui_appearance', 'youtube', 'content_filter', 'itunes', 'm3u_export', 'musicbrainz', 'deezer', 'audiodb', 'metadata', 'hydrabase', 'security', 'discogs', 'library', 'discover', 'wishlist', 'genre_whitelist']:
                 if service in new_settings:
                     for key, value in new_settings[service].items():
                         config_manager.set(f'{service}.{key}', value)