fix: Spotify lyrics validation and proactive prefetching

- Only attempt Spotify lyrics for tracks with valid Spotify IDs (22 chars, no 'local' or ':')
- Add Spotify IDs to external matched tracks in playlists for lyrics support
- Proactively fetch and cache lyrics when playback starts (background task)
- Fix pre-existing SubSonicController bug (missing _cache field)
- Lyrics now ready instantly when requested by client

allstarr/Controllers/SubSonicController.cs

@@ -28,6 +28,7 @@ public class SubsonicController : ControllerBase
     private readonly SubsonicModelMapper _modelMapper;
     private readonly SubsonicProxyService _proxyService;
     private readonly PlaylistSyncService? _playlistSyncService;
+    private readonly RedisCacheService _cache;
     private readonly ILogger<SubsonicController> _logger;
     
     public SubsonicController(
@@ -39,6 +40,7 @@ public class SubsonicController : ControllerBase
         SubsonicResponseBuilder responseBuilder,
         SubsonicModelMapper modelMapper,
         SubsonicProxyService proxyService,
+        RedisCacheService cache,
         ILogger<SubsonicController> logger,
         PlaylistSyncService? playlistSyncService = null)
     {
@@ -51,6 +53,7 @@ public class SubsonicController : ControllerBase
         _modelMapper = modelMapper;
         _proxyService = proxyService;
         _playlistSyncService = playlistSyncService;
+        _cache = cache;
         _logger = logger;
 
         if (string.IsNullOrWhiteSpace(_subsonicSettings.Url))