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/Domain/Album.cs

@@ -0,0 +1,20 @@
+namespace octo_fiesta.Models.Domain;
+
+/// <summary>
+/// Represents an album
+/// </summary>
+public class Album
+{
+    public string Id { get; set; } = string.Empty;
+    public string Title { get; set; } = string.Empty;
+    public string Artist { get; set; } = string.Empty;
+    public string? ArtistId { get; set; }
+    public int? Year { get; set; }
+    public int? SongCount { get; set; }
+    public string? CoverArtUrl { get; set; }
+    public string? Genre { get; set; }
+    public bool IsLocal { get; set; }
+    public string? ExternalProvider { get; set; }
+    public string? ExternalId { get; set; }
+    public List<Song> Songs { get; set; } = new();
+}