- Replace sd.wait() with a poll loop + sd.stop() in play_beep().
sd.wait() calls Event.wait() without timeout — hangs forever if the
audio device stalls. Poll with a 2s ceiling and force-stop instead.
- Wrap _on_silence callback in try-except so exceptions are logged
instead of silently lost in the daemon thread. Prevents recording
state from becoming inconsistent on unexpected errors.