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/Musify.Worker.Dockerfile

@@ -17,18 +17,9 @@ RUN cp /app/AppSettings.json /app/appsettings.json \
 
 FROM mcr.microsoft.com/dotnet/aspnet:10.0 AS runtime
 WORKDIR /app
-# ffmpeg: the Worker transcodes audio to .m4a. yt-dlp: downloads YouTube audio.
-# yt-dlp is installed via pip in a venv rather than the standalone PyInstaller
-# binary: the binary adds a ~1s unpack cost per invocation and extracts slower,
-# so pip roughly halves resolution latency. deno solves nsig challenges.
+# ffmpeg: the Worker transcodes uploaded audio to .m4a.
 RUN apt-get update \
-    && apt-get install -y --no-install-recommends ffmpeg python3 python3-venv ca-certificates curl unzip \
-    && python3 -m venv /opt/ytdlp \
-    && /opt/ytdlp/bin/pip install --no-cache-dir -U yt-dlp \
-    && ln -s /opt/ytdlp/bin/yt-dlp /usr/local/bin/yt-dlp \
-    && curl -fsSL https://deno.land/install.sh | sh -s -- -y \
-    && mv /root/.deno/bin/deno /usr/local/bin/deno \
-    && apt-get purge -y curl unzip \
-    && rm -rf /var/lib/apt/lists/* /root/.deno
+    && apt-get install -y --no-install-recommends ffmpeg ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
 COPY --from=build /app ./
 ENTRYPOINT ["dotnet", "Musify.Worker.dll"]