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/Configuration/MixConfiguration.cs

@@ -6,16 +6,7 @@ namespace Musify.Application.Configuration
     {
         public const string SectionName = "Mix";
 
-        [Range(1, 20)]
-        public int ArtistMixCount { get; set; } = 3;
-
         [Range(4, 100)]
         public int ItemsPerMix { get; set; } = 12;
-
-        [Range(1, 10)]
-        public int SeedArtistCount { get; set; } = 3;
-
-        [Range(1, 100)]
-        public int YouTubeResultsPerSeed { get; set; } = 12;
     }
 }