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/Hosts/Musify.Api/AppSettings.json

@@ -53,21 +53,8 @@
         "AnonymousFragmentSeconds": 0,
         "EstimatedAudioBytesPerSecond": 16000
     },
-    "YouTube": {
-        "Enabled": true,
-        "GeographicalLocation": "US",
-        "StreamUrlCacheSeconds": 1800,
-        "SearchContinuationCacheSeconds": 600,
-        "SearchResultsCacheSeconds": 900,
-        "SongInfoCacheSeconds": 86400,
-        "ThumbnailSize": 240,
-        "ArtworkSize": 544
-    },
     "Mix": {
-        "ArtistMixCount": 3,
-        "ItemsPerMix": 12,
-        "SeedArtistCount": 3,
-        "YouTubeResultsPerSeed": 12
+        "ItemsPerMix": 12
     },
     "PlayList": {
         "Routes": {