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/core/alerts.py

@@ -495,9 +495,10 @@ async def _dispatch_notification(
     event_id: str,
     note: str | None = None,
 ) -> None:
-    """Run after the transaction commits. Holds the alert's email for its
-    burst (core.alert_burst), posts its webhook, and records the webhook's
-    outcome on the event; the burst records the email's."""
+    """Run after the transaction commits. Queues the alert's email in the
+    outbox, gathered for 2 minutes with any others (core.outbox.hold_alert),
+    posts its webhook, and records the webhook's outcome on the event; the
+    outbox records the email's once it settles."""
     from steward.core.notifications import dispatch_notifications
 
     alert_data = {
@@ -513,11 +514,14 @@ async def _dispatch_notification(
         "timestamp": datetime.now(timezone.utc).isoformat(),
     }
 
-    # Email waits a moment so alerts that arrive together go out as one
-    # message per recipient (#5465); the webhook posts this alert now.
-    from steward.core import alert_burst
+    # Email waits so alerts that arrive together go out as one message per
+    # recipient (#5465, #5507); the webhook posts this alert now.
+    from steward.core import outbox
     if app.config.get("SMTP", {}).get("host"):
-        alert_burst.hold(app, alert_data, event_id)
+        try:
+            await outbox.hold_alert(app, alert_data, event_id)
+        except Exception:
+            logger.exception("Alert %s: its email could not be queued", event_id)
     results = await dispatch_notifications({}, app.config.get("WEBHOOK", {}), alert_data)
 
     webhook_r = results.get("webhook")