Follow-up to the off-loop move: once the credential-pool handlers run on
worker threads, the dashboard's periodic /api/credentials/pool polls can
overlap, and during a DNS outage each poll would have started its own
exchange and abandoned its own hung resolver thread.
- Per-fingerprint threading.Lock around the exchange: concurrent callers
wait on the one in-flight attempt, then hit the positive or negative
cache (bounded worker count, no duplicate network calls).
- _urlopen_bounded: when the hard cap fires and the abandoned worker later
succeeds, close the HTTPResponse instead of leaking the socket.
- Tests (none shipped with the original PR): hard cap + late-close,
single-flight success and failure paths, and the pool endpoint running
off-loop / keeping the loop responsive under a 200 ms blocking read.
a3d33fe22f