Drain the repository purge outbox: all versions + delete markers per prefix.

A repository removal commits its S3 work here rather than running it in the request (a large repo is hundreds of builds x objects x versions). Always 2xx: the launcher fails the rest of its reconcile tick on any non-2xx, and a prefix that cannot be purged is a counted row plus a >24h WARNING, never a failed tick.

First removes repositories GitHub reported gone whose grace has passed (app/services/repository_gone.py), so their purge rows drain in the same tick. Piggybacks here so no launcher change is needed.