YouTube now returns "Sign in to confirm you're not a bot" for yt-dlp from
datacenter IPs, blocking downloads, search and link-resolve. When a cookies file
is present (YTDLP_COOKIES, default /data/youtube-cookies.txt) every yt-dlp call
passes --cookies so it authenticates. Checked at call time (drop the file in, no
code change); absent → unchanged behaviour. Applied to the download cascade, the
YTM search probe, and the importer resolve.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>