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/Channels/SRFPlayChannel.cs

@@ -190,7 +190,7 @@ public class SRFPlayChannel : IChannel, IHasCacheKey
         {
             try
             {
-                var businessUnit = config.BusinessUnit.ToString().ToLowerInvariant();
+                var businessUnit = config.BusinessUnit.ToLowerString();
                 var topics = await _categoryService.GetTopicsAsync(businessUnit, cancellationToken).ConfigureAwait(false);
 
                 foreach (var topic in topics.Where(t => !string.IsNullOrEmpty(t.Id)))
@@ -248,7 +248,7 @@ public class SRFPlayChannel : IChannel, IHasCacheKey
 
         try
         {
-            var businessUnit = config?.BusinessUnit.ToString().ToLowerInvariant() ?? "srf";
+            var businessUnit = config?.BusinessUnit.ToLowerString() ?? "srf";
 
             using var apiClient = _apiClientFactory.CreateClient();
             var scheduledLivestreams = await apiClient.GetScheduledLivestreamsAsync(businessUnit, "SPORT", cancellationToken).ConfigureAwait(false);
@@ -310,7 +310,7 @@ public class SRFPlayChannel : IChannel, IHasCacheKey
         {
             var config = Plugin.Instance?.Configuration;
             var topicId = folderId.Substring("category_".Length);
-            var businessUnit = config?.BusinessUnit.ToString().ToLowerInvariant() ?? "srf";
+            var businessUnit = config?.BusinessUnit.ToLowerString() ?? "srf";
 
             var shows = await _categoryService.GetShowsByTopicAsync(topicId, businessUnit, 20, cancellationToken).ConfigureAwait(false);
             var urns = new List<string>();
@@ -440,7 +440,7 @@ public class SRFPlayChannel : IChannel, IHasCacheKey
                 _logger.LogDebug("Processing URN: {Urn}", urn);
                 var mediaComposition = await apiClient.GetMediaCompositionByUrnAsync(urn, cancellationToken).ConfigureAwait(false);
 
-                if (mediaComposition?.ChapterList == null || mediaComposition.ChapterList.Count == 0)
+                if (mediaComposition?.HasChapters != true)
                 {
                     _logger.LogWarning("URN {Urn}: No media composition or chapters found", urn);
                     failedCount++;