Remove YouTube integration from backend and deployment config

Drops the YouTube-backed second catalog source entirely: the
Local/External split on Track/Album collapses into single concrete
entities, Artist/TrackArtist/ExternalTrack/ExternalAlbum and the
YouTube service/download pipeline (yt-dlp, MassTransit consumer,
routing slip) are deleted, and mixes now source discovery only from
local listening history. Includes the RemoveYouTubeIntegration EF
Core migration and matching Worker/deploy/compose cleanup (no more
yt-dlp/python/deno in the Worker image, no thumbnail-cache/YtDlp
config sections).

backend/Core/Musify.Application/Contracts/IDatabase.cs

@@ -11,22 +11,10 @@ namespace Musify.Application.Contracts
 
         DbSet<Album> Albums { get; }
 
-        DbSet<UserAlbum> UserAlbums { get; }
-
-        DbSet<ExternalAlbum> ExternalAlbums { get; }
-
         DbSet<AlbumHasTrack> AlbumHasTracks { get; }
 
         DbSet<Track> Tracks { get; }
 
-        DbSet<LocalTrack> LocalTracks { get; }
-
-        DbSet<ExternalTrack> ExternalTracks { get; }
-
-        DbSet<Artist> Artists { get; }
-
-        DbSet<TrackArtist> TrackArtists { get; }
-
         DbSet<UserHasTrack> UserHasTracks { get; }
 
         DbSet<PlayListHasTrack> PlayListHasTracks { get; }

backend/Core/Musify.Application/Contracts/IYouTubeDownloaderService.cs

@@ -1,9 +0,0 @@
-namespace Musify.Application.Contracts
-{
-    public record YouTubeDownloadResult(int ExitCode, string StandardError);
-
-    public interface IYouTubeDownloaderService
-    {
-        Task<YouTubeDownloadResult> DownloadAudioAsync(string videoId, string destinationFilePath, CancellationToken cancellationToken);
-    }
-}

backend/Core/Musify.Application/Contracts/IYouTubeMusicService.cs

@@ -1,64 +0,0 @@
-using ErrorOr;
-
-namespace Musify.Application.Contracts
-{
-    public record YouTubeArtistRef(string? Id, string Name);
-
-    public record YouTubeSongResult(
-        string VideoId,
-        string Title,
-        string Artist,
-        string Album,
-        int DurationSeconds,
-        string ThumbnailUrl,
-        bool IsExplicit,
-        IReadOnlyList<YouTubeArtistRef> Artists);
-
-    public record YouTubeSearchResult(
-        IReadOnlyList<YouTubeSongResult> Items,
-        string ContinuationToken);
-
-    public record YouTubeAlbumResult(
-        string AlbumId,
-        string Title,
-        string Artist,
-        string ThumbnailUrl,
-        int? ReleaseYear,
-        bool IsSingle,
-        bool IsEp,
-        IReadOnlyList<YouTubeArtistRef> Artists);
-
-    public record YouTubeAlbumSearchResult(
-        IReadOnlyList<YouTubeAlbumResult> Items,
-        string ContinuationToken);
-
-    public record YouTubeAlbumTrack(
-        string VideoId,
-        string Title,
-        int DurationSeconds,
-        int TrackNumber,
-        bool IsExplicit);
-
-    public record YouTubeAlbumDetail(
-        YouTubeAlbumResult Album,
-        string Description,
-        int TotalDurationSeconds,
-        IReadOnlyList<YouTubeAlbumTrack> Tracks);
-
-    public record YouTubeStreamInfo(string Url, int ExpiresInSeconds);
-
-    public interface IYouTubeMusicService
-    {
-        Task<ErrorOr<YouTubeSearchResult>> SearchSongsAsync(string query, string continuationToken, CancellationToken cancellationToken);
-
-        Task<ErrorOr<YouTubeAlbumSearchResult>> SearchAlbumsAsync(string query, string continuationToken, CancellationToken cancellationToken);
-
-        Task<ErrorOr<YouTubeAlbumDetail>> GetAlbumAsync(string albumId, CancellationToken cancellationToken);
-
-        Task<ErrorOr<YouTubeStreamInfo>> GetAudioStreamAsync(string videoId, CancellationToken cancellationToken);
-
-        Task<ErrorOr<YouTubeSongResult>> GetSongAsync(string videoId, CancellationToken cancellationToken);
-
-        string ResolveArtworkUrl(string thumbnailUrl);
-    }
-}