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/Controllers/AdminController.cs

@@ -1738,6 +1738,30 @@ public class AdminController : ControllerBase
         });
     }
     
+    /// <summary>
+    /// Clear all cached lyrics (LRCLIB only - Jellyfin and Spotify lyrics are not cached)
+    /// </summary>
+    [HttpPost("cache/clear-lyrics")]
+    public async Task<IActionResult> ClearLyricsCache()
+    {
+        _logger.LogInformation("Lyrics cache clear requested from admin UI");
+        
+        var clearedRedisKeys = 0;
+        
+        // Clear all lyrics cache keys (pattern-based deletion)
+        // This includes LRCLIB lyrics and manual mappings
+        var lyricsKeysDeleted = await _cache.DeleteByPatternAsync("lyrics:*");
+        clearedRedisKeys += lyricsKeysDeleted;
+        
+        _logger.LogInformation("Lyrics cache cleared: {RedisKeys} Redis keys deleted", clearedRedisKeys);
+        
+        return Ok(new { 
+            message = "Lyrics cache cleared successfully", 
+            redisKeysDeleted = clearedRedisKeys,
+            note = "Only LRCLIB lyrics are cached. Jellyfin and Spotify lyrics are fetched on-demand (fast)."
+        });
+    }
+    
     /// <summary>
     /// Restart the allstarr container to apply configuration changes
     /// </summary>