List recipient ids attached to a monitor

GET/PUT responses always carry both recipient_ids and recipients (resolved from the same snapshot), so a client reading either field sees the attached set. PUT request bodies accept exactly one of recipient_ids (today's form) or recipients (issue #338, per-pair down_delay_s) — specifying both, or specifying neither, is 400. Either field, when present, must not be JSON null (400) — only an explicit [] detaches every recipient. Omitting down_delay_s on a recipients entry keeps an already-attached pair's stored delay, or defaults a newly-attached pair to 0; the recipient_ids form always keeps every attached pair's existing delay.