- Add payment_request.cancelled listener that reverts member/dependent to
  payment_pending when their payment is cancelled from the queue
- Allow cancellation of completed payment requests (voids linked payment)
- Add cancel button for completed items in queue UI
- Fix self-healing logic to not re-activate dependents whose payment was
  intentionally cancelled (checks for cancelled requests without a valid
  completed replacement)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>