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/Albums/AddTrackToAlbum.cs

@@ -17,7 +17,7 @@ namespace Musify.Application.Albums
     {
         public async ValueTask<ErrorOr<Success>> Handle(AddTrackToAlbumCommand request, CancellationToken cancellationToken)
         {
-            var album = await database.UserAlbums
+            var album = await database.Albums
                 .AsNoTracking()
                 .SingleOrDefaultAsync(a => a.Id == request.AlbumId, cancellationToken);
             if (album is null)
@@ -32,19 +32,16 @@ namespace Musify.Application.Albums
                 return Error.Unauthorized();
             }
 
-            var trackExists = await database.Tracks
+            var track = await database.Tracks
                 .AsNoTracking()
-                .AnyAsync(t => t.Id == request.TrackId, cancellationToken);
-            if (!trackExists)
+                .SingleOrDefaultAsync(t => t.Id == request.TrackId, cancellationToken);
+            if (track is null)
             {
                 logger.LogInformation("Track {TrackId} not found", request.TrackId);
                 return Error.NotFound(description: $"Track {request.TrackId} not found");
             }
 
-            var localTrack = await database.LocalTracks
-                .AsNoTracking()
-                .SingleOrDefaultAsync(t => t.Id == request.TrackId, cancellationToken);
-            if (localTrack is not null && localTrack.OwnerUserId != request.UserId)
+            if (track.OwnerUserId != request.UserId)
             {
                 logger.LogWarning("User {UserId} does not own track {TrackId}", request.UserId, request.TrackId);
                 return Error.Unauthorized();