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.Infrastructure/Configuration/YouTubeConfiguration.cs

@@ -1,35 +0,0 @@
-using System.ComponentModel.DataAnnotations;
-
-namespace Musify.Infrastructure.Configuration
-{
-    public class YouTubeConfiguration
-    {
-        public const string SectionName = "YouTube";
-
-        /// <summary>Turns the whole YouTube Music source on or off: search, albums, streaming
-        /// and adding new YouTube tracks all fail with a clear error while this is <c>false</c>,
-        /// and mix generation silently skips YouTube candidates instead of failing.</summary>
-        public bool Enabled { get; set; } = true;
-
-        [Required]
-        public string GeographicalLocation { get; set; } = "US";
-
-        [Range(60, 21600)]
-        public int StreamUrlCacheSeconds { get; set; } = 1800;
-
-        [Range(60, 3600)]
-        public int SearchContinuationCacheSeconds { get; set; } = 600;
-
-        [Range(0, 86400)]
-        public int SearchResultsCacheSeconds { get; set; } = 900;
-
-        [Range(0, 604800)]
-        public int SongInfoCacheSeconds { get; set; } = 86400;
-
-        [Range(32, 720)]
-        public int ThumbnailSize { get; set; } = 240;
-
-        [Range(120, 1080)]
-        public int ArtworkSize { get; set; } = 544;
-    }
-}

backend/Core/Musify.Infrastructure/Configuration/YtDlpConfiguration.cs

@@ -1,20 +0,0 @@
-using System.ComponentModel.DataAnnotations;
-
-namespace Musify.Infrastructure.Configuration
-{
-    public class YtDlpConfiguration
-    {
-        public const string SectionName = "YtDlp";
-
-        [Required]
-        public string ExecutableName { get; set; } = "yt-dlp";
-
-        [Required]
-        public string Format { get; set; } = "bestaudio";
-
-        public string AdditionalArguments { get; set; } = string.Empty;
-
-        [Required]
-        public TimeSpan DownloadTimeout { get; set; } = TimeSpan.FromMinutes(5);
-    }
-}