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>

tests/core/test_alert_history_row.py

@@ -16,7 +16,7 @@ RULE = SimpleNamespace(metric_name="cpu_pct", operator=AlertOperator(">"), thres
 def _event(**kw):
     base = dict(transitioned_at=datetime(2026, 10, 6, tzinfo=timezone.utc), from_state="inactive",
                 to_state="firing", metric_value=97.5, note=None, email_sent=None, email_error=None,
-                webhook_sent=None, webhook_error=None, ansible_run_id=None)
+                webhook_sent=None, webhook_error=None, ansible_run_id=None, actor_id=None)
     return SimpleNamespace(**{**base, **kw})
 
 
@@ -48,3 +48,16 @@ def test_only_attempted_deliveries_are_listed():
     row = _history_row(_event(email_sent=True, webhook_sent=True), RULE)
     assert [(label, sent) for label, sent, _ in row["deliveries"]] == [("email", True), ("webhook", True)]
     assert _history_row(_event(), RULE)["deliveries"] == []
+
+
+def test_a_transition_a_person_made_says_who():
+    """#5463: an acknowledgement, and a rule change that reset the alert,
+    name the person; one whose account is gone still reads as a person's."""
+    names = {"u1": "bob"}
+    ack = _event(from_state="firing", to_state="acknowledged", metric_value=None, actor_id="u1")
+    assert _history_row(ack, RULE, names)["notes"] == ["acknowledged by bob"]
+    off = _event(from_state="firing", to_state="resolved", note="rule disabled", actor_id="u1")
+    assert _history_row(off, RULE, names)["notes"] == ["rule disabled by bob"]
+    gone = _event(from_state="firing", to_state="acknowledged", actor_id="u9")
+    assert _history_row(gone, RULE, names)["notes"] == ["acknowledged by a deleted user"]
+    assert _history_row(_event(note="rule enabled"), RULE, names)["notes"] == ["rule enabled"]