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/Mixes/Responses/MixItemApplicationResponse.cs

@@ -1,27 +1,8 @@
-using Musify.Domain.Entities;
-using Musify.Domain.ValueObjects;
-
 namespace Musify.Application.Mixes.Responses
 {
     public record MixItemApplicationResponse(
-        MixItemSource Source,
-        Guid? TrackId,
-        string? VideoId,
+        Guid TrackId,
         string Title,
         string? Artist,
-        string? ThumbnailUrl,
-        double DurationSeconds,
-        bool IsExplicit)
-    {
-        public static MixItemApplicationResponse FromEntity(MixItem item) =>
-            new(
-                item.Source,
-                item.TrackId,
-                item.VideoId,
-                item.Title ?? string.Empty,
-                item.Artist,
-                item.ThumbnailUrl,
-                item.DurationSeconds,
-                item.IsExplicit);
-    }
+        double DurationSeconds);
 }