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/Settings/QobuzSettings.cs

@@ -0,0 +1,25 @@
+namespace octo_fiesta.Models.Settings;
+
+/// <summary>
+/// Configuration for the Qobuz downloader and metadata service
+/// </summary>
+public class QobuzSettings
+{
+    /// <summary>
+    /// Qobuz user authentication token
+    /// Obtained from browser's localStorage after logging into play.qobuz.com
+    /// </summary>
+    public string? UserAuthToken { get; set; }
+    
+    /// <summary>
+    /// Qobuz user ID
+    /// Obtained from browser's localStorage after logging into play.qobuz.com
+    /// </summary>
+    public string? UserId { get; set; }
+    
+    /// <summary>
+    /// Preferred audio quality: FLAC, MP3_320, MP3_128
+    /// If not specified or unavailable, the highest available quality will be used.
+    /// </summary>
+    public string? Quality { get; set; }
+}