Fix authentication issues in SpotifyTrackMatchingService

- Fixed SpotifyTrackMatchingService to use GetJsonAsyncInternal for authenticated requests
- This resolves 401 Unauthorized errors when fetching existing playlist tracks
- Should prevent unnecessary rematching when cache is warm
- Fixes 'No Items found in Jellyfin playlist response' warnings

The service was using GetJsonAsync with null headers, causing 401 errors.
Now uses server API key authentication for internal operations.

allstarr/Services/Spotify/SpotifyTrackMatchingService.cs

@@ -241,19 +241,19 @@ public class SpotifyTrackMatchingService : BackgroundService
                     // CRITICAL: Must include UserId parameter or Jellyfin returns empty results
                     var userId = jellyfinSettings.UserId;
                     var playlistItemsUrl = $"Playlists/{playlistConfig.JellyfinId}/Items";
+                    var queryParams = new Dictionary<string, string>();
                     if (!string.IsNullOrEmpty(userId))
                     {
-                        playlistItemsUrl += $"?UserId={userId}";
+                        queryParams["UserId"] = userId;
                     }
                     else
                     {
                         _logger.LogWarning("No UserId configured - may not be able to fetch existing playlist tracks for {Playlist}", playlistName);
                     }
                     
-                    var (existingTracksResponse, _) = await proxyService.GetJsonAsync(
+                    var (existingTracksResponse, _) = await proxyService.GetJsonAsyncInternal(
                         playlistItemsUrl, 
-                        null, 
-                        null);
+                        queryParams);
                     
                     if (existingTracksResponse != null && 
                         existingTracksResponse.RootElement.TryGetProperty("Items", out var items))