CI: automatically re-download a PR's metadata once its CI finishes

This is perhaps the last remaining blocker to leanprover-community/queueboard#70 (which is part of leanprover-community/queueboard-core#23).

If a PR's CI finishes, github does not register this as an "update": this means that as-is, this repo's data will not automate directly. (It will re-download this after 60 minutes, but this adds a sizable delay.) A better solution is for the failing CI run to notify this repository.

This can be implemented through a github webhook. This webhook would call gather_stats_single.yml (caution: that logic has diverged from download_missing_outdated_prs.sh, make sure to synchronise before landing). Perhaps this repository needs to be transferred to the leanprover-community organisation for this to work.

My knowledge mostly ends here: help with this would be very warmly welcome.

Start by reading gather_stats_single.yml and download_missing_outdated_prs.sh, then compare their diverged logic. Investigate how a GitHub webhook could invoke the workflow when a PR's CI finishes and whether repository transfer is required. Done means synchronized download logic runs without waiting for the 60-minute refresh.