The OOK subprocess was spawned without start_new_session=True, so
process.terminate() only signalled the parent — child processes kept
running. Now uses os.killpg() to terminate the entire process group,
matching the pattern used by all other routes (ADS-B, AIS, ACARS, etc.).
Also fixes silent error swallowing in the frontend stop handler so the
UI resets even if the backend request fails.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>