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/Constants/ApiEndpoints.cs

@@ -28,12 +28,6 @@ public static class ApiEndpoints
     /// </summary>
     public const string SrfPlayHomepage = "https://www.srf.ch/play";
 
-    /// <summary>
-    /// Media composition endpoint template (relative to IntegrationLayerBaseUrl).
-    /// Format: {0} = URN.
-    /// </summary>
-    public const string MediaCompositionByUrnPath = "/mediaComposition/byUrn/{0}.json";
-
     /// <summary>
     /// Base proxy route for stream proxying (relative to server root).
     /// </summary>
@@ -44,30 +38,4 @@ public static class ApiEndpoints
     /// Format: {0} = item ID.
     /// </summary>
     public const string ProxyMasterManifestPath = "/Plugins/SRFPlay/Proxy/{0}/master.m3u8";
-
-    /// <summary>
-    /// Image proxy route template (relative to server root).
-    /// Format: {0} = base64-encoded original URL.
-    /// </summary>
-    public const string ImageProxyPath = "/Plugins/SRFPlay/Image/{0}";
-
-    /// <summary>
-    /// Play V3 shows endpoint (relative to PlayV3 base URL).
-    /// </summary>
-    public const string PlayV3ShowsPath = "shows";
-
-    /// <summary>
-    /// Play V3 topics endpoint (relative to PlayV3 base URL).
-    /// </summary>
-    public const string PlayV3TopicsPath = "topics";
-
-    /// <summary>
-    /// Play V3 livestreams endpoint (relative to PlayV3 base URL).
-    /// </summary>
-    public const string PlayV3LivestreamsPath = "livestreams";
-
-    /// <summary>
-    /// Play V3 scheduled livestreams endpoint (relative to PlayV3 base URL).
-    /// </summary>
-    public const string PlayV3ScheduledLivestreamsPath = "scheduled-livestreams";
 }