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/MediaComposition.cs

@@ -1,5 +1,4 @@
 using System.Collections.Generic;
-using System.Collections.ObjectModel;
 using System.Text.Json.Serialization;
 
 namespace Jellyfin.Plugin.SRFPlay.Api.Models;
@@ -15,6 +14,12 @@ public class MediaComposition
     [JsonPropertyName("chapterList")]
     public IReadOnlyList<Chapter> ChapterList { get; set; } = new List<Chapter>();
 
+    /// <summary>
+    /// Gets a value indicating whether this composition has any chapters.
+    /// </summary>
+    [JsonIgnore]
+    public bool HasChapters => ChapterList != null && ChapterList.Count > 0;
+
     /// <summary>
     /// Gets or sets the episode information.
     /// </summary>