Mark interaction signals as reviewed. This is the acknowledgement behind the Activity To Do block: an item of type=interest (a new pairing or a phase change on this profile) disappears once acknowledged, and the event will not be re-sent by a notifier. Body: {"event_ids": [...]} for one item, or {"all": true} for the rollup item. Events are scoped to the entity that session owns: acknowledging signals of a different entity is refused (ids not belonging to this entity are ignored).