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; }