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>

docs/core/alerting.md

@@ -184,6 +184,14 @@ seconds and settles each email on the server's answer:
 Once every email carrying an alert has settled, that alert's event records
 whether the email went, and to whom it didn't.
 
+Where to see it: each alert's row in **Alerts → History** says what its email
+is doing (gathering, with when it sends; retrying, with the attempt, the next
+try and the server's last answer; sent; failed, with who wasn't reached and
+why), and `get_alert_history` over MCP reports the same. **Settings → System →
+Outgoing email** lists every email of the last 7 days not yet sent, failures
+first, and the page's verdict names failures and any email still retrying
+after 5 minutes.
+
 An email a person is waiting on is tried at once instead of on the next tick:
 the weekly report's **Send now**, an invite, and the test buttons, so the page
 can say what the server answered. A report or invite the server couldn't take