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/Services/Interfaces/IMetadataCache.cs

@@ -22,20 +22,8 @@ public interface IMetadataCache
     /// <param name="mediaComposition">The media composition to cache.</param>
     void SetMediaComposition(string urn, MediaComposition mediaComposition);
 
-    /// <summary>
-    /// Removes media composition from cache.
-    /// </summary>
-    /// <param name="urn">The URN.</param>
-    void RemoveMediaComposition(string urn);
-
     /// <summary>
     /// Clears all cached data.
     /// </summary>
     void Clear();
-
-    /// <summary>
-    /// Gets the cache statistics.
-    /// </summary>
-    /// <returns>A tuple with cache count and size estimate.</returns>
-    (int Count, long SizeEstimate) GetStatistics();
 }