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.Domain/Entities/MixItem.cs

@@ -1,8 +1,6 @@
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
 
-using Musify.Domain.ValueObjects;
-
 namespace Musify.Domain.Entities
 {
 #pragma warning disable CS8618
@@ -19,32 +17,12 @@ namespace Musify.Domain.Entities
         public int Position { get; set; } = 0;
 
         [Required]
-        public required MixItemSource Source { get; set; }
-
-        public Guid? TrackId { get; set; }
-
-        [MaxLength(64)]
-        public string? VideoId { get; set; }
-
-        [MaxLength(300)]
-        public string? Title { get; set; }
-
-        [MaxLength(300)]
-        public string? Artist { get; set; }
-
-        [MaxLength(512)]
-        public string? ThumbnailUrl { get; set; }
-
-        [Required]
-        public double DurationSeconds { get; set; }
-
-        [Required]
-        public bool IsExplicit { get; set; }
+        public required Guid TrackId { get; set; }
 
         [ForeignKey(nameof(MixId))]
         public Mix Mix { get; set; }
 
         [ForeignKey(nameof(TrackId))]
-        public Track? Track { get; set; }
+        public Track Track { get; set; }
     }
 }