Return a curated digest of actions taken on trip_id by someone other than the requesting traveler, newest first. Each item states whether the action could be undone when it was taken and whether it can still be undone — clients should gate an undo affordance on isStillReversible, not on isReversible. since bounds the window and defaults to seven days ago; it must be a timezone-aware instant. This is a synthesis, not a page of the timeline: internal state-propagation events are excluded and the result is capped rather than paginated. 404 when the trip does not exist; 403 when it belongs to a different user.