Finalize Run

Run end-of-meeting Brevo list operations for a completed run.

Called as a Cloud Task from the meeting.ended webhook (immediately) and from the handle-webinar-end safety net. Per-step run.brevo_*_at timestamps inside run_end_of_run_lifecycle make this idempotent — successful steps are skipped on retry.

Expected JSON body: {"run_id": <int>}