new core/video/youtube_download.py: fetch a wished youtube video end to end.
plans the organised dest (channel/year/date template), builds yt-dlp opts from
the quality profile (format_selection), runs the download (injectable factory),
then on success marks the video_downloads row completed + archives to history +
removes it from the wishlist; on failure archives failed and KEEPS the wish to
retry. orchestration is pure (yt-dlp run + all db writes injected) + seam-tested;
run_youtube_download binds the real seams for the worker thread.
flows through the SAME video_downloads queue as movies/tv (model B) so the
downloads page + history work for youtube for free.
