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/email/alert.txt

@@ -0,0 +1,20 @@
+{# An alert email's plain-text part; the same facts as alert.html. #}
+{{ a.state }}: {{ a.headline }}
+
+Rule: {{ a.rule }}
+Resource: {{ a.resource }}
+Reading: {{ a.reading }}
+Fires when: {{ a.condition }}
+When: {{ a.when }}{% if a.when_utc %} ({{ a.when_utc }}){% endif %}
+
+{% if a.note %}
+Note: {{ a.note }}
+{% endif %}
+{% if link %}
+
+View in Steward: {{ link }}
+{% endif %}
+
+--
+Steward sends this to everyone on its email list when an alert rule changes
+state. Who gets it is set in Settings › Notifications.