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/SRFMediaProvider.cs

@@ -70,7 +70,7 @@ public class SRFMediaProvider : IMediaSourceProvider
 
             var mediaComposition = await _compositionFetcher.GetMediaCompositionAsync(urn, cancellationToken, cacheDuration).ConfigureAwait(false);
 
-            if (mediaComposition?.ChapterList == null || mediaComposition.ChapterList.Count == 0)
+            if (mediaComposition?.HasChapters != true)
             {
                 _logger.LogWarning("No chapters found for URN: {Urn}", urn);
                 return sources;
@@ -116,7 +116,7 @@ public class SRFMediaProvider : IMediaSourceProvider
                 // Force fresh fetch with short cache duration
                 var freshMediaComposition = await _compositionFetcher.GetMediaCompositionAsync(urn, cancellationToken, 1).ConfigureAwait(false);
 
-                if (freshMediaComposition?.ChapterList != null && freshMediaComposition.ChapterList.Count > 0)
+                if (freshMediaComposition?.HasChapters == true)
                 {
                     var freshChapter = freshMediaComposition.ChapterList[0];
                     mediaSource = await _mediaSourceFactory.CreateMediaSourceAsync(