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.html

@@ -0,0 +1,27 @@
+{# An alert email's HTML part: a = mail.alert_facts(), link = the alerts page or "". #}
+{% extends "_layout.html" %}
+{% from "_parts.html" import badge, button, fact with context %}
+{% block title %}{{ a.state }}: {{ a.rule }}{% endblock %}
+{% block body %}
+<tr><td style="padding:14px 24px 0 24px;">{{ badge(a.tone, a.state) }}</td></tr>
+<tr><td style="padding:10px 24px 4px 24px;font:600 20px/1.35 {{ c.font }};color:{{ c.text }};">{{ a.headline }}</td></tr>
+<tr><td style="padding:10px 24px 4px 24px;">
+<table role="presentation" width="100%" cellpadding="0" cellspacing="0" border="0">
+{{ fact("Rule", a.rule) }}
+{{ fact("Resource", a.resource) }}
+{{ fact("Reading", a.reading) }}
+{{ fact("Fires when", a.condition) }}
+{% call fact("When") %}{{ a.when }}{% if a.when_utc %}<br><span style="color:{{ c.muted }};">{{ a.when_utc }}</span>{% endif %}{% endcall %}
+{% if a.note %}{{ fact("Note", a.note) }}{% endif %}
+</table>
+</td></tr>
+{% if link %}
+<tr><td style="padding:16px 24px 24px 24px;">{{ button(link, "View in Steward") }}</td></tr>
+{% else %}
+<tr><td style="padding:0 0 12px 0;"></td></tr>
+{% endif %}
+{% endblock %}
+{% block footer %}
+Steward sends this to everyone on its email list when an alert rule changes state.
+Who gets it is set in Settings › Notifications.
+{% endblock %}