Closing the terminal or pressing Ctrl-\ killed the reloader and left the pool parent holding the port while it respawned workers that could not start. The pool now stops once its parent reloader is gone. The reload pool also defaults the worker healthcheck to 30 seconds unless --timeout_worker_healthcheck is set, since uvicorn's 5 second default killed workers that were still booting on a reload.
Workers now load the app through a factory that exits with uvicorn's
STARTUP_FAILURE code on any import error, so the pool stops and the
reloader waits for the next save, the same as single-worker reload.
Multi-worker reload needs uvicorn>=0.50.0 for that, so older versions
keep the single-worker reload with a warning.
