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/Song.cs

@@ -0,0 +1,97 @@
+namespace octo_fiesta.Models.Domain;
+
+/// <summary>
+/// Represents a song (local or external)
+/// </summary>
+public class Song
+{
+    /// <summary>
+    /// Unique ID. For external songs, prefixed with "ext-" + provider + "-" + external id
+    /// Example: "ext-deezer-123456" or "local-789"
+    /// </summary>
+    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 string Album { get; set; } = string.Empty;
+    public string? AlbumId { get; set; }
+    public int? Duration { get; set; } // In seconds
+    public int? Track { get; set; }
+    public int? DiscNumber { get; set; }
+    public int? TotalTracks { get; set; }
+    public int? Year { get; set; }
+    public string? Genre { get; set; }
+    public string? CoverArtUrl { get; set; }
+    
+    /// <summary>
+    /// High-resolution cover art URL (for embedding)
+    /// </summary>
+    public string? CoverArtUrlLarge { get; set; }
+    
+    /// <summary>
+    /// BPM (beats per minute) if available
+    /// </summary>
+    public int? Bpm { get; set; }
+    
+    /// <summary>
+    /// ISRC (International Standard Recording Code)
+    /// </summary>
+    public string? Isrc { get; set; }
+    
+    /// <summary>
+    /// Full release date (format: YYYY-MM-DD)
+    /// </summary>
+    public string? ReleaseDate { get; set; }
+    
+    /// <summary>
+    /// Album artist name (may differ from track artist)
+    /// </summary>
+    public string? AlbumArtist { get; set; }
+    
+    /// <summary>
+    /// Composer(s)
+    /// </summary>
+    public string? Composer { get; set; }
+    
+    /// <summary>
+    /// Album label
+    /// </summary>
+    public string? Label { get; set; }
+    
+    /// <summary>
+    /// Copyright
+    /// </summary>
+    public string? Copyright { get; set; }
+    
+    /// <summary>
+    /// Contributing artists (features, etc.)
+    /// </summary>
+    public List<string> Contributors { get; set; } = new();
+    
+    /// <summary>
+    /// Indicates whether the song is available locally or needs to be downloaded
+    /// </summary>
+    public bool IsLocal { get; set; }
+    
+    /// <summary>
+    /// External provider (deezer, spotify, etc.) - null if local
+    /// </summary>
+    public string? ExternalProvider { get; set; }
+    
+    /// <summary>
+    /// ID on the external provider (for downloading)
+    /// </summary>
+    public string? ExternalId { get; set; }
+    
+    /// <summary>
+    /// Local file path (if available)
+    /// </summary>
+    public string? LocalPath { get; set; }
+    
+    /// <summary>
+    /// Deezer explicit content lyrics value
+    /// 0 = Naturally clean, 1 = Explicit, 2 = Not applicable, 3 = Clean/edited version, 6/7 = Unknown
+    /// </summary>
+    public int? ExplicitContentLyrics { get; set; }
+}