refactor: reorganize Models into subdirectories by context

Split monolithic MusicModels.cs (177 lines) into separate files:
- Models/Settings/ (DeezerSettings, QobuzSettings, SubsonicSettings)
- Models/Domain/ (Song, Album, Artist)
- Models/Search/ (SearchResult)
- Models/Download/ (DownloadInfo, DownloadStatus)
- Models/Subsonic/ (ScanStatus)

Updated namespaces and imports across 22 files.
Improves navigation and separates models by business context.

octo-fiesta/Models/Download/DownloadStatus.cs

@@ -0,0 +1,12 @@
+namespace octo_fiesta.Models.Download;
+
+/// <summary>
+/// Download status of a song
+/// </summary>
+public enum DownloadStatus
+{
+    NotStarted,
+    InProgress,
+    Completed,
+    Failed
+}