feat(email): alert and weekly-report emails read as designed messages (#5335)

#5335 "Emails Steward sends read as designed messages: an HTML alert email
and weekly report, with a plain-text twin".

- core/mail.py renders each email as multipart/alternative from
  templates/email/: a table-and-inline-style HTML part and a plain-text
  twin carrying the same facts. Colours are the design system's light
  theme, resolved from theme.css; nothing is fetched when a mail opens.
- Alert email: state badge, a headline sentence ("Disk used on web01 is
  94%."), rule, resource, reading, the condition in words, and the time in
  the zone Notifications was saved in with UTC beside it; "View in
  Steward" when Steward's address is set. Subject "[Steward] Firing: <rule>
  — <resource> 94%". Alerts now carry their operator.
- Weekly report: monitors down, active alerts, the week's alert events,
  uptime and busiest Traefik routers as small tables with links. Sent from
  the configured From address; the "Generated … UTC" line and the stale
  repo footer are gone.
- condition_text() states a condition from a rule's parts.

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

steward/templates/settings/notifications.html

@@ -56,6 +56,10 @@
       <input type="text" name="smtp.recipients"
              value="{{ settings['smtp.recipients'] | join(', ') }}"
              placeholder="admin@example.com, ops@example.com">
+      <p style="font-size:0.78rem;color:var(--text-muted);margin-top:0.25rem;">
+        Alert emails give times in {{ tz_label() }}, your zone when you save, with UTC beside them.
+        {% if not settings['general.public_base_url'] %}Set Steward's address in <a href="/settings/">General</a> and they link back here.{% endif %}
+      </p>
     </div>
 
     <button type="button" class="btn btn-ghost" hx-post="/settings/notifications/test-email"