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/Resource.cs

@@ -2,6 +2,9 @@ using System.Text.Json.Serialization;
 
 namespace Jellyfin.Plugin.SRFPlay.Api.Models;
 
+// NOTE: DrmList is typed as object? because the SRF API returns either null or a JSON array.
+// IsPlayable checks for both null and empty array ("[]") to determine if content is DRM-free.
+
 /// <summary>
 /// Represents a streaming resource (URL) in the SRF API response.
 /// </summary>
@@ -48,4 +51,10 @@ public class Resource
     /// </summary>
     [JsonPropertyName("drmList")]
     public object? DrmList { get; set; }
+
+    /// <summary>
+    /// Gets a value indicating whether this resource is playable (not DRM-protected).
+    /// </summary>
+    [JsonIgnore]
+    public bool IsPlayable => DrmList == null || DrmList.ToString() == "[]";
 }