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/Subsonic/ScanStatus.cs

@@ -0,0 +1,10 @@
+namespace octo_fiesta.Models.Subsonic;
+
+/// <summary>
+/// Subsonic library scan status
+/// </summary>
+public class ScanStatus
+{
+    public bool Scanning { get; set; }
+    public int? Count { get; set; }
+}