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

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+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; }
+}

octo-fiesta/Models/Settings/QobuzSettings.cs

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+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; }
+}

octo-fiesta/Models/Settings/SubsonicSettings.cs

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+namespace octo_fiesta.Models.Settings;
+
+/// <summary>
+/// Download mode for tracks
+/// </summary>
+public enum DownloadMode
+{
+    /// <summary>
+    /// Download only the requested track (default behavior)
+    /// </summary>
+    Track,
+    
+    /// <summary>
+    /// When a track is played, download the entire album in background
+    /// The requested track is downloaded first, then remaining tracks are queued
+    /// </summary>
+    Album
+}
+
+/// <summary>
+/// Explicit content filter mode for Deezer tracks
+/// </summary>
+public enum ExplicitFilter
+{
+    /// <summary>
+    /// Show all tracks (no filtering)
+    /// </summary>
+    All,
+    
+    /// <summary>
+    /// Exclude clean/edited versions (explicit_content_lyrics == 3)
+    /// Shows original explicit content and naturally clean content
+    /// </summary>
+    ExplicitOnly,
+    
+    /// <summary>
+    /// Only show clean content (explicit_content_lyrics == 0 or 3)
+    /// Excludes tracks with explicit_content_lyrics == 1
+    /// </summary>
+    CleanOnly
+}
+
+/// <summary>
+/// Music service provider
+/// </summary>
+public enum MusicService
+{
+    /// <summary>
+    /// Deezer music service
+    /// </summary>
+    Deezer,
+    
+    /// <summary>
+    /// Qobuz music service
+    /// </summary>
+    Qobuz
+}
+
+public class SubsonicSettings
+{
+    public string? Url { get; set; }
+    
+    /// <summary>
+    /// Explicit content filter mode (default: All)
+    /// Environment variable: EXPLICIT_FILTER
+    /// Values: "All", "ExplicitOnly", "CleanOnly"
+    /// Note: Only works with Deezer
+    /// </summary>
+    public ExplicitFilter ExplicitFilter { get; set; } = ExplicitFilter.All;
+    
+    /// <summary>
+    /// Download mode for tracks (default: Track)
+    /// Environment variable: DOWNLOAD_MODE
+    /// Values: "Track" (download only played track), "Album" (download full album when playing a track)
+    /// </summary>
+    public DownloadMode DownloadMode { get; set; } = DownloadMode.Track;
+    
+    /// <summary>
+    /// Music service to use (default: Deezer)
+    /// Environment variable: MUSIC_SERVICE
+    /// Values: "Deezer", "Qobuz"
+    /// </summary>
+    public MusicService MusicService { get; set; } = MusicService.Deezer;
+}