Several "container hangs after adding a printer" reports (#1486) share a
signature with nothing to act on: HTTP goes silent, /health hangs, the
process may ignore SIGTERM, and the log just stops mid-stream - a frozen
asyncio loop cannot log a thing.
loop_watchdog re-arms faulthandler.dump_traceback_later() from an async
heartbeat. While the loop ticks the timer is cancelled and re-armed before
it can fire; if the loop stalls for 30s the heartbeat can't re-arm and
faulthandler's C-level timer thread dumps every thread's stack to stderr,
so the blocked frame shows up in `docker compose logs`.
