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.html

@@ -0,0 +1,63 @@
+{# The weekly report's HTML part: d = reports.build_report_data(), alerts = its
+   active alerts with tone and label, period, sent, base = Steward's address or "". #}
+{% extends "_layout.html" %}
+{% from "_parts.html" import badge, button, section, rows, row with context %}
+{% block title %}Weekly report, {{ period }}{% endblock %}
+{% block body %}
+<tr><td style="padding:10px 24px 0 24px;font:600 22px/1.3 {{ c.font }};color:{{ c.text }};">Weekly report</td></tr>
+<tr><td style="padding:2px 24px 0 24px;font:14px/1.5 {{ c.font }};color:{{ c.muted }};">{{ period }}</td></tr>
+
+{% if d.hosts_down %}
+<tr><td style="padding:16px 24px 0 24px;">
+<table role="presentation" width="100%" cellpadding="0" cellspacing="0" border="0"
+       style="border:1px solid {{ c.crit }};border-left:4px solid {{ c.crit }};border-radius:8px;">
+<tr><td style="padding:10px 14px;font:14px/1.5 {{ c.font }};color:{{ c.text }};">
+<strong style="color:{{ c.crit }};">{{ d.hosts_down|length }} {{ "monitor is" if d.hosts_down|length == 1 else "monitors are" }} down now:</strong>
+{{ d.hosts_down|join(", ") }}
+</td></tr></table>
+</td></tr>
+{% endif %}
+
+{{ section("Active alerts", base ~ "/alerts/" if base else "") }}
+{% if alerts %}
+{% call rows() %}
+{% for a in alerts %}
+{{ row([badge(a.tone, a.state), a.name ~ " — " ~ a.resource, a.condition]) }}
+{% endfor %}
+{% endcall %}
+{% else %}
+<tr><td style="padding:0 24px;font:14px/1.5 {{ c.font }};color:{{ c.muted }};">None. Nothing is firing.</td></tr>
+{% endif %}
+<tr><td style="padding:8px 24px 0 24px;font:13px/1.5 {{ c.font }};color:{{ c.muted }};">
+This week {{ d.alert_events.fired }} {{ "alert" if d.alert_events.fired == 1 else "alerts" }} fired and {{ d.alert_events.resolved }} resolved.
+</td></tr>
+
+{{ section("Uptime over 7 days", base ~ "/monitors/" if base else "") }}
+{% if d.uptime_summary %}
+{% call rows() %}
+{% for m in d.uptime_summary %}
+{{ row([m.name, "—" if m.pct_7d is none else "%.2f%%"|format(m.pct_7d)]) }}
+{% endfor %}
+{% endcall %}
+{% else %}
+<tr><td style="padding:0 24px;font:14px/1.5 {{ c.font }};color:{{ c.muted }};">No monitors are enabled.</td></tr>
+{% endif %}
+
+{% if d.top_routers %}
+{{ section("Busiest Traefik routers, last 24 hours", base ~ "/plugins/traefik/" if base else "") }}
+{% call rows() %}
+{% for r in d.top_routers %}
+{{ row([r.name, "%.2f req/s"|format(r.avg_rate)]) }}
+{% endfor %}
+{% endcall %}
+{% endif %}
+
+{% if base %}
+<tr><td style="padding:24px 24px 24px 24px;">{{ button(base ~ "/", "Open Steward") }}</td></tr>
+{% else %}
+<tr><td style="padding:0 0 16px 0;"></td></tr>
+{% endif %}
+{% endblock %}
+{% block footer %}
+Sent {{ sent }}. The weekly report is turned on, and its day and hour set, in Settings › Reports.
+{% endblock %}