Fix lyrics fetching and disable prefetching

- Fix LyricsPrefetchService to use server API key for Jellyfin lyrics checks
- Remove Spotify lyrics caching (local Docker container is fast)
- Disable lyrics prefetching service (not needed - Jellyfin/Spotify are fast)
- Add POST /api/admin/cache/clear-lyrics endpoint to clear LRCLIB cache
- Only LRCLIB lyrics are cached now (external API)

allstarr/Services/Lyrics/SpotifyLyricsService.cs

@@ -63,15 +63,7 @@ public class SpotifyLyricsService
         // Normalize track ID (remove URI prefix if present)
         spotifyTrackId = ExtractTrackId(spotifyTrackId);
         
-        // Check cache
-        var cacheKey = $"spotify:lyrics:{spotifyTrackId}";
-        var cached = await _cache.GetAsync<SpotifyLyricsResult>(cacheKey);
-        if (cached != null)
-        {
-            _logger.LogDebug("Returning cached Spotify lyrics for track {TrackId}", spotifyTrackId);
-            return cached;
-        }
-        
+        // NO CACHING - Spotify lyrics come from local Docker container (fast)
         try
         {
             var url = $"{_settings.LyricsApiUrl}/?trackid={spotifyTrackId}&format=id3";
@@ -92,8 +84,6 @@ public class SpotifyLyricsService
             
             if (result != null)
             {
-                // Cache for 30 days (lyrics don't change)
-                await _cache.SetAsync(cacheKey, result, TimeSpan.FromDays(30));
                 _logger.LogInformation("Got Spotify lyrics from sidecar for track {TrackId} ({LineCount} lines)", 
                     spotifyTrackId, result.Lines.Count);
             }