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.Development.json

@@ -36,9 +36,6 @@
     "OpenTelemetry": {
         "OtlpEndpoint": "http://localhost:4317"
     },
-    "YouTube": {
-        "GeographicalLocation": "ES"
-    },
     "Workers": {
         "Routes": {
             "TemporaryFilesDirectory": "../../../deploy/.tmp"