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/templates/email/burst.txt

@@ -0,0 +1,18 @@
+{# Several alerts that arrived together, plain text; the same facts as burst.html. #}
+{{ alerts|length }} alerts changed together: {{ said }}.
+{% for a in alerts %}
+
+{{ a.state }}: {{ a.headline }}
+  {{ a.rule }} · fires when {{ a.condition }} · {{ a.when }}
+{%- if a.note %}
+  {{ a.note }}
+{%- endif %}
+{% endfor %}
+{% if link %}
+
+View in Steward: {{ link }}
+{% endif %}
+
+--
+Steward sends this to everyone on its email list when alert rules change
+state. Who gets it is set in Settings › Notifications.