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).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/Core/Musify.Application/Tracks/DeleteTrack.cs

@@ -29,7 +29,7 @@ namespace Musify.Application.Tracks
                 return Error.NotFound();
             }
 
-            if (track is not LocalTrack localTrack || localTrack.OwnerUserId != request.UserId)
+            if (track.OwnerUserId != request.UserId)
             {
                 logger.LogWarning("User {UserId} unauthorized to delete track {TrackId}", request.UserId, request.TrackId);
                 return Error.Unauthorized();