feat(email): delivery you can see: each alert's email in history, outgoing email on Settings -> System (#5509)

#5509 "Delivery you can see: each alert's email state in history, and
outgoing email on Settings -> System" (milestone 501).

- Alerts -> History: an alert's email reads from the outbox (#5506):
  "gathering" with when it sends, "sending", "retrying" with the attempt,
  next try and the server's last answer, "sent", or "failed" naming each
  reader it didn't reach and why (outbox.delivery / email_states). Events
  from before the outbox keep what they recorded. The Notified column
  shows once an alert has a queued email.
- Settings -> System gets "Outgoing email": every email of the last 7
  days not yet sent, failures first, then the longest retrying, with
  recipient, kind, state and last error, and a count of what was sent.
  Failures and anything still retrying after 5 minutes lead the page's
  verdict (outbox.outgoing).
- get_alert_history (MCP) adds `email`: the same delivery state.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/alerts/service.py

@@ -322,9 +322,12 @@ async def history_columns(db, since: datetime, rule_id: str | None = None) -> tu
     base = select(AlertEvent.id).where(AlertEvent.transitioned_at >= since)
     if rule_id:
         base = base.where(AlertEvent.rule_id == rule_id)
+    from steward.models.outbox import email_outbox_events
+    queued = select(email_outbox_events.c.event_id).where(
+        email_outbox_events.c.event_id == AlertEvent.id).exists()
     notified = await db.scalar(select(base.where(or_(
         AlertEvent.email_sent.is_not(None), AlertEvent.webhook_sent.is_not(None),
-        AlertEvent.note.like(f"%{MUTED}%"))).exists()))
+        AlertEvent.note.like(f"%{MUTED}%"), queued)).exists()))
     acted = await db.scalar(select(base.where(AlertEvent.ansible_run_id.is_not(None)).exists()))
     return bool(notified), bool(acted)