feat(alerts): alert email waits 2 minutes in the outbox and each alert records its outcome (#5507)

#5507 "Alert emails through the outbox, gathered for 2 minutes"
(milestone 501). Replaces the in-memory core/alert_burst.py.

- outbox.hold_alert: an alert joins each recipient's held alert email, or
  opens one due 2 minutes later (System 156 ruling). A Postgres advisory
  lock keeps two alerts at once from opening two windows for one reader.
- email_outbox_events.alert_json (migration 0043) keeps the alert as it
  fired, so the email written when the window closes says what it said.
- write_held writes it then, in the reader's zone: the usual alert email
  for one alert, the list for several (outbox.alert_words).
- after_settle records on each AlertEvent, once every email carrying it
  has settled: email_sent, or email_error naming each reader it didn't
  reach and the server's reason.
- A held email survives a restart and goes out when its window closes.
- A refusal of a message's one recipient no longer repeats the address.

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

steward/templates/email/burst.html

@@ -1,4 +1,4 @@
-{# Several alerts that arrived together (core.alert_burst, #5465), as one
+{# Several alerts that arrived together (core.outbox.alert_words, #5465), as one
    email's HTML part: alerts = [mail.alert_facts()], oldest first; said = how
    many of each ("3 firing, 1 resolved"); link = the alerts page or "". #}
 {% extends "_layout.html" %}