fix: make PlexMetadata type/key/title nullable for folder items

Plex API returns Directory entries without type field, causing
TypeError crash in fromJson. Made fields nullable and migrated
callers to use mediaType enum and displayTitle getter instead
of raw string access.

lib/utils/deletion_notifier.dart

@@ -80,7 +80,7 @@ class DeletionNotifier extends BaseNotifier<DeletionEvent> {
         ratingKey: metadata.ratingKey,
         serverId: metadata.serverId ?? '',
         parentChain: _buildParentChain(metadata),
-        mediaType: metadata.type,
+        mediaType: metadata.type ?? '',
         leafCount: metadata.leafCount ?? 1,
         isDownloadOnly: isDownloadOnly,
       ),