- PurgeSiteCacheAfterCommit queues the purge on a per-purger worker after
commit: one request in flight, saves committed meanwhile collapse into
one follow-up, so the admin request never waits for the site
- new cooler.SiteCachePurger.Wait drains the queue; Handle.Close waits for
it at shutdown (timeout plus one second)
- Purge stays the synchronous send; exported names and signatures unchanged