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/DeezerSettings.cs

@@ -0,0 +1,25 @@
+namespace octo_fiesta.Models.Settings;
+
+/// <summary>
+/// Configuration for the Deezer downloader and metadata service
+/// </summary>
+public class DeezerSettings
+{
+    /// <summary>
+    /// Deezer ARL token (required for downloading)
+    /// Obtained from browser cookies after logging into deezer.com
+    /// </summary>
+    public string? Arl { get; set; }
+    
+    /// <summary>
+    /// Fallback ARL token (optional)
+    /// Used if the primary ARL token fails
+    /// </summary>
+    public string? ArlFallback { 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; }
+}