Clean up dead code, consolidate duplication, fix redundancies

Remove 9 dead methods, 6 unused constants, and redundant
ReaderWriterLockSlim from MetadataCache. Consolidate repeated
patterns into HasChapters, IsPlayable, and ToLowerString helpers.
Extract shared API methods in SRFApiClient. Move variant manifest
rewriting from controller to StreamProxyService. Make Auto quality
distinct from HD. Update README architecture section.

Jellyfin.Plugin.SRFPlay/Providers/SRFEpisodeProvider.cs

@@ -50,7 +50,7 @@ public class SRFEpisodeProvider : IRemoteMetadataProvider<Episode, EpisodeInfo>
 
                 var mediaComposition = await _compositionFetcher.GetMediaCompositionAsync(urn, cancellationToken).ConfigureAwait(false);
 
-                if (mediaComposition?.ChapterList != null && mediaComposition.ChapterList.Count > 0)
+                if (mediaComposition?.HasChapters == true)
                 {
                     var chapter = mediaComposition.ChapterList[0];
                     results.Add(new RemoteSearchResult
@@ -93,7 +93,7 @@ public class SRFEpisodeProvider : IRemoteMetadataProvider<Episode, EpisodeInfo>
 
             var mediaComposition = await _compositionFetcher.GetMediaCompositionAsync(urn, cancellationToken).ConfigureAwait(false);
 
-            if (mediaComposition?.ChapterList == null || mediaComposition.ChapterList.Count == 0)
+            if (mediaComposition?.HasChapters != true)
             {
                 _logger.LogWarning("No chapter information found for URN: {Urn}", urn);
                 return result;