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/Utilities/Extensions.cs

@@ -1,5 +1,4 @@
 using Jellyfin.Plugin.SRFPlay.Configuration;
-using MediaBrowser.Controller.Entities;
 
 namespace Jellyfin.Plugin.SRFPlay.Utilities;
 
@@ -8,18 +7,6 @@ namespace Jellyfin.Plugin.SRFPlay.Utilities;
 /// </summary>
 public static class Extensions
 {
-    /// <summary>
-    /// Gets the SRF URN from the item's provider IDs.
-    /// </summary>
-    /// <param name="item">The base item.</param>
-    /// <returns>The SRF URN, or null if not found or empty.</returns>
-    public static string? GetSrfUrn(this BaseItem item)
-    {
-        return item.ProviderIds.TryGetValue("SRF", out var urn) && !string.IsNullOrEmpty(urn)
-            ? urn
-            : null;
-    }
-
     /// <summary>
     /// Converts the BusinessUnit enum to its lowercase string representation.
     /// </summary>
@@ -29,13 +16,4 @@ public static class Extensions
     {
         return businessUnit.ToString().ToLowerInvariant();
     }
-
-    /// <summary>
-    /// Gets the plugin configuration safely, returning null if not available.
-    /// </summary>
-    /// <returns>The plugin configuration, or null if not available.</returns>
-    public static PluginConfiguration? GetPluginConfig()
-    {
-        return Plugin.Instance?.Configuration;
-    }
 }