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/MixApplicationResponse.cs

@@ -1,5 +1,3 @@
-using Musify.Domain.Entities;
-
 namespace Musify.Application.Mixes.Responses
 {
     public record MixApplicationResponse(
@@ -10,16 +8,5 @@ namespace Musify.Application.Mixes.Responses
         IReadOnlyList<MixItemApplicationResponse> Items)
     {
         public const int CoverItemCount = 4;
-
-        public static MixApplicationResponse FromEntity(
-            Mix mix,
-            int itemCount,
-            IReadOnlyList<MixItem> items) =>
-            new(
-                mix.Id,
-                mix.Title,
-                mix.Subtitle,
-                itemCount,
-                [.. items.Select(MixItemApplicationResponse.FromEntity)]);
     }
 }