feat(alerts): alerts that arrive together go out as one email (#5465)

#5465 "One email per burst of alerts". When a switch reboots, every host's
agent-offline rule fires at once and each account got one email per rule.
Alert email now waits 30 s from the first alert: each recipient then gets one
message listing every alert in the window, oldest first, in their own zone
("[Steward] 3 alerts: 2 firing, 1 resolved"). One alert alone keeps its
usual email. Webhooks still post per alert, at once. Every alert's event
records the email's outcome. A burst held when Steward stops is not sent;
the alerts stay on the record and on the Alerts page.

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

steward/core/alerts.py

@@ -495,7 +495,9 @@ async def _dispatch_notification(
     event_id: str,
     note: str | None = None,
 ) -> None:
-    """Run after the transaction commits. Sends notifications and updates the event row."""
+    """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."""
     from steward.core.notifications import dispatch_notifications
 
     alert_data = {
@@ -511,29 +513,23 @@ async def _dispatch_notification(
         "timestamp": datetime.now(timezone.utc).isoformat(),
     }
 
-    from steward.core.mail import recipients
-    smtp_cfg = app.config.get("SMTP", {})
-    webhook_cfg = app.config.get("WEBHOOK", {})
-    async with app.db_sessionmaker() as db:
-        to = await recipients(db, "alerts", smtp_cfg) if smtp_cfg.get("host") else []
-    results = await dispatch_notifications(smtp_cfg, webhook_cfg, alert_data, to)
+    # 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
+    if app.config.get("SMTP", {}).get("host"):
+        alert_burst.hold(app, alert_data, event_id)
+    results = await dispatch_notifications({}, app.config.get("WEBHOOK", {}), alert_data)
 
-    # Update event row with delivery results in a new transaction
+    webhook_r = results.get("webhook")
+    if not webhook_r:
+        return
     async with app.db_sessionmaker() as db:
         async with db.begin():
-            event_result = await db.execute(
-                select(AlertEvent).where(AlertEvent.id == event_id)
-            )
-            event = event_result.scalar_one_or_none()
+            event = (await db.execute(
+                select(AlertEvent).where(AlertEvent.id == event_id))).scalar_one_or_none()
             if event:
-                email_r = results.get("email")
-                if email_r:
-                    event.email_sent = email_r["sent"]
-                    event.email_error = email_r.get("error")
-                webhook_r = results.get("webhook")
-                if webhook_r:
-                    event.webhook_sent = webhook_r["sent"]
-                    event.webhook_error = webhook_r.get("error")
+                event.webhook_sent = webhook_r["sent"]
+                event.webhook_error = webhook_r.get("error")
 
 
 async def _run_ansible_action(