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.Worker/AppSettings.json

@@ -26,14 +26,8 @@
     "ApplicationStorage": {
         "Bucket": ""
     },
-    "YouTube": {
-        "Enabled": true
-    },
     "Mix": {
-        "ArtistMixCount": 3,
-        "ItemsPerMix": 12,
-        "SeedArtistCount": 3,
-        "YouTubeResultsPerSeed": 12
+        "ItemsPerMix": 12
     },
     "PlayList": {
         "Routes": {
@@ -83,12 +77,6 @@
         },
         "TranscodingTimeout": "00:05:00"
     },
-    "YtDlp": {
-        "ExecutableName": "yt-dlp",
-        "Format": "bestaudio",
-        "AdditionalArguments": "",
-        "DownloadTimeout": "00:05:00"
-    },
     "Workers": {
         "Routes": {
             "TemporaryFilesDirectory": ""