After each successful indexing run, the source's download_cutoff_date is
automatically advanced to 7 days ago if the current cutoff date is older
(or nil). This prevents yt-dlp from scanning through months of old videos
on every index run, significantly improving indexing performance for
channels with large backlogs.
The 7-day buffer ensures recent videos are still checked on subsequent
index runs while keeping indexing fast (minutes instead of hours).
Also updates AGENTS.md to document Docker-based development workflow
and adds Pinchflat_improvements.md for tracking known issues.
