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/IStreamProxyService.cs

@@ -8,15 +8,6 @@ namespace Jellyfin.Plugin.SRFPlay.Services.Interfaces;
 /// </summary>
 public interface IStreamProxyService
 {
-    /// <summary>
-    /// Registers a stream for proxying with an already-authenticated URL.
-    /// </summary>
-    /// <param name="itemId">The item ID.</param>
-    /// <param name="authenticatedUrl">The authenticated stream URL.</param>
-    /// <param name="urn">The SRF URN for this content (used for re-fetching fresh URLs).</param>
-    /// <param name="isLiveStream">Whether this is a livestream (livestreams always fetch fresh URLs).</param>
-    void RegisterStream(string itemId, string authenticatedUrl, string? urn = null, bool isLiveStream = false);
-
     /// <summary>
     /// Registers a stream for deferred authentication (authenticates on first playback request).
     /// Use this when browsing to avoid wasting 30-second tokens before the user clicks play.
@@ -61,6 +52,15 @@ public interface IStreamProxyService
     /// <returns>The segment content as bytes.</returns>
     Task<byte[]?> GetSegmentAsync(string itemId, string segmentPath, string? queryString = null, CancellationToken cancellationToken = default);
 
+    /// <summary>
+    /// Rewrites URLs in a variant (sub) manifest to point to the proxy.
+    /// </summary>
+    /// <param name="manifestContent">The variant manifest content.</param>
+    /// <param name="baseProxyUrl">The base proxy URL (without query params).</param>
+    /// <param name="queryParams">Query parameters to append to rewritten URLs (e.g., "?token=abc").</param>
+    /// <returns>The rewritten manifest content.</returns>
+    string RewriteVariantManifestUrls(string manifestContent, string baseProxyUrl, string queryParams);
+
     /// <summary>
     /// Cleans up old and expired stream mappings.
     /// </summary>