feat(alerts): an acknowledgement is on the record, with who made it (#5463)

#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>

steward/models/alerts.py

@@ -124,6 +124,12 @@ class AlertEvent(Base):
     email_error: Mapped[str | None] = mapped_column(Text, nullable=True)
     webhook_sent: Mapped[bool | None] = mapped_column(Boolean, nullable=True)
     webhook_error: Mapped[str | None] = mapped_column(Text, nullable=True)
+    # Who made this transition happen, when a person did: an acknowledgement,
+    # or a rule edit, disable or enable that reset the alert (#5463). None for
+    # every transition a sample or Steward itself caused.
+    actor_id: Mapped[str | None] = mapped_column(
+        String(36), ForeignKey("users.id", ondelete="SET NULL"), nullable=True
+    )
     # Set when this firing event triggered an Ansible action run (audit link).
     ansible_run_id: Mapped[str | None] = mapped_column(
         String(36), ForeignKey("ansible_runs.id", ondelete="SET NULL"), nullable=True