feat: spotify playlist injection

- Add SpotifyImportSettings configuration model
- Create SpotifyMissingTracksFetcher background service
- Inject virtual Spotify playlists into search results
- Auto-match tracks from external providers
- Update README with feature documentation
- Configure sync window and playlist settings

allstarr/Program.cs

@@ -108,6 +108,8 @@ builder.Services.Configure<SquidWTFSettings>(
     builder.Configuration.GetSection("SquidWTF"));
 builder.Services.Configure<RedisSettings>(
     builder.Configuration.GetSection("Redis"));
+builder.Services.Configure<SpotifyImportSettings>(
+    builder.Configuration.GetSection("SpotifyImport"));
 
 // Get shared settings from the active backend config
 MusicService musicService;
@@ -229,6 +231,9 @@ builder.Services.AddHostedService<StartupValidationOrchestrator>();
 // Register cache cleanup service (only runs when StorageMode is Cache)
 builder.Services.AddHostedService<CacheCleanupService>();
 
+// Register Spotify missing tracks fetcher (only runs when SpotifyImport is enabled)
+builder.Services.AddHostedService<allstarr.Services.Spotify.SpotifyMissingTracksFetcher>();
+
 builder.Services.AddCors(options =>
 {
     options.AddDefaultPolicy(policy =>