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/report.txt

@@ -0,0 +1,35 @@
+{# The weekly report's plain-text part; the same facts as report.html. #}
+Steward weekly report, {{ period }}
+
+{% if d.hosts_down %}
+{{ d.hosts_down|length }} {{ "monitor is" if d.hosts_down|length == 1 else "monitors are" }} down now: {{ d.hosts_down|join(", ") }}
+
+{% endif %}
+ACTIVE ALERTS{% if base %} ({{ base }}/alerts/){% endif %}
+
+{% for a in alerts %}
+- {{ a.state }}: {{ a.name }} — {{ a.resource }} ({{ a.condition }})
+{% else %}
+None. Nothing is firing.
+{% endfor %}
+This week {{ d.alert_events.fired }} {{ "alert" if d.alert_events.fired == 1 else "alerts" }} fired and {{ d.alert_events.resolved }} resolved.
+
+UPTIME OVER 7 DAYS{% if base %} ({{ base }}/monitors/){% endif %}
+
+{% for m in d.uptime_summary %}
+- {{ m.name }}: {{ "—" if m.pct_7d is none else "%.2f%%"|format(m.pct_7d) }}
+{% else %}
+No monitors are enabled.
+{% endfor %}
+{% if d.top_routers %}
+
+BUSIEST TRAEFIK ROUTERS, LAST 24 HOURS{% if base %} ({{ base }}/plugins/traefik/){% endif %}
+
+{% for r in d.top_routers %}
+- {{ r.name }}: {{ "%.2f req/s"|format(r.avg_rate) }}
+{% endfor %}
+{% endif %}
+
+--
+Sent {{ sent }}. The weekly report is turned on, and its day and hour set,
+in Settings › Reports.