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/Api/Models/Chapter.cs

@@ -74,7 +74,9 @@ public class Chapter
     /// Gets or sets the list of available resources (streams).
     /// </summary>
     [JsonPropertyName("resourceList")]
-    public IReadOnlyList<Resource> ResourceList { get; set; } = new List<Resource>();
+    [System.Diagnostics.CodeAnalysis.SuppressMessage("Design", "CA1002:Do not expose generic lists", Justification = "Required for JSON deserialization")]
+    [System.Diagnostics.CodeAnalysis.SuppressMessage("Design", "CA2227:Collection properties should be read only", Justification = "Required for JSON deserialization")]
+    public List<Resource> ResourceList { get; set; } = new List<Resource>();
 
     /// <summary>
     /// Gets or sets the episode number.