fix: add authentication to playlist cache pre-building

- PreBuildPlaylistItemsCacheAsync was failing with HTTP 401
- Background services don't have client headers for authentication
- Now manually creates X-Emby-Authorization header with API key
- Fixes 'Failed to fetch Jellyfin playlist items: HTTP 401' warning
- Playlist items cache now builds successfully after track matching

All 225 tests pass.

allstarr/Services/Spotify/SpotifyTrackMatchingService.cs

@@ -3,6 +3,7 @@ using allstarr.Models.Settings;
 using allstarr.Models.Spotify;
 using allstarr.Services.Common;
 using allstarr.Services.Jellyfin;
+using Microsoft.AspNetCore.Http;
 using Microsoft.Extensions.Options;
 using System.Text.Json;
 
@@ -701,8 +702,15 @@ public class SpotifyTrackMatchingService : BackgroundService
                 return;
             }
             
+            // Create authentication headers for background service call
+            var headers = new HeaderDictionary();
+            if (!string.IsNullOrEmpty(jellyfinSettings.ApiKey))
+            {
+                headers["X-Emby-Authorization"] = $"MediaBrowser Token=\"{jellyfinSettings.ApiKey}\"";
+            }
+            
             var playlistItemsUrl = $"Playlists/{jellyfinPlaylistId}/Items?UserId={userId}&Fields=MediaSources";
-            var (existingTracksResponse, statusCode) = await proxyService.GetJsonAsync(playlistItemsUrl, null, null);
+            var (existingTracksResponse, statusCode) = await proxyService.GetJsonAsync(playlistItemsUrl, null, headers);
             
             if (statusCode != 200 || existingTracksResponse == null)
             {