- Preview workers ignore SIGINT (pool mode only) and load tracerite, so
Ctrl-C no longer dumps a KeyboardInterrupt traceback per worker.
- Spawn workers in their own process group and kill with killpg, so a
SIGKILLed worker cannot orphan an in-flight ffmpeg grandchild.
- Fail all pending and in-flight preview futures when the pool closes
instead of orphaning them until timeout; never restart the pool once
shut down (mid-shutdown requests get a quiet 'preview cancelled' 503).
- Re-raise CancelledError in the preview route instead of responding on
a torn-down connection ('NoneType' is_closing crash).
- Log preview failures with logger.exception where they occur (in the
worker, whose stderr is inherited) instead of re-logging a traceback-
less error string in the parent.