feat: configurable intro/credits marker regex patterns

Add settings to customize regex patterns for matching intro and credits
markers in chapter titles, with the current hardcoded values as defaults.
Also reclassifies real markers with non-standard type strings against the
configured patterns.

lib/services/plex_client.dart

@@ -954,7 +954,7 @@ class PlexClient {
 
   /// Get chapters and markers from cached metadata or fetch if needed
   /// Uses same cache key as other metadata methods for consistency
-  Future<PlaybackExtras> getPlaybackExtras(String ratingKey) async {
+  Future<PlaybackExtras> getPlaybackExtras(String ratingKey, {String? introPattern, String? creditsPattern}) async {
     try {
       final data = await _fetchWithCacheFirst<Map<String, dynamic>>(
         cacheKey: '/library/metadata/$ratingKey',
@@ -964,7 +964,7 @@ class PlexClient {
         parseResponse: (response) => response.data as Map<String, dynamic>?,
       );
       final metadataJson = _getFirstMetadataJsonFromData(data);
-      return _parsePlaybackExtrasFromMetadataJson(metadataJson);
+      return _parsePlaybackExtrasFromMetadataJson(metadataJson, introPattern: introPattern, creditsPattern: creditsPattern);
     } catch (e) {
       appLogger.w('Failed to get playback extras', error: e);
       return PlaybackExtras(chapters: [], markers: []);
@@ -972,10 +972,12 @@ class PlexClient {
   }
 
   /// Parse PlaybackExtras from metadata JSON
-  PlaybackExtras _parsePlaybackExtrasFromMetadataJson(Map<String, dynamic>? metadataJson) {
+  PlaybackExtras _parsePlaybackExtrasFromMetadataJson(Map<String, dynamic>? metadataJson, {String? introPattern, String? creditsPattern}) {
     return PlaybackExtras.withChapterFallback(
       chapters: _parseChapters(metadataJson),
       markers: _parseMarkers(metadataJson),
+      introPatternStr: introPattern,
+      creditsPatternStr: creditsPattern,
     );
   }