#5463 "Acknowledgements on record: an event and who did it", step 1 of milestone 499 "Alerts you can trust". The live walk in #5461 found that acknowledging changed only the alert's current state: once it resolved, nothing said who had acknowledged it or when. - alert_events.actor_id (migration 0040): the person behind a transition. - Acknowledging writes a firing → acknowledged event by the acknowledger. - A rule edit, disable or enable that resets an alert carries its actor (web and MCP alike). - The history page notes "acknowledged by X" and "rule disabled by X"; MCP get_alert_history returns `by`. - One usernames() helper in alerts.service, used by the active list, the history page and the MCP tools. - docs/core/alerting.md: the state diagram no longer shows an acknowledged alert re-firing, and says what an event records. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>