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/alerts/service.py

@@ -134,15 +134,21 @@ _BINARY = ("is_up", "is_healthy")
 def rule_condition(rule: AlertRule) -> str:
     """What a rule fires on, in words: "CPU > 90%", "Fullest disk >= 85%",
     or "is down" for an up/down metric rather than "is_up < 0.5"."""
+    return condition_text(rule.metric_name, rule.operator, rule.threshold)
+
+
+def condition_text(metric: str, operator: AlertOperator | str, threshold: float) -> str:
+    """rule_condition from a rule's parts, for an alert carried as a dict (an
+    email); the operator may be its value, ">=", rather than the enum."""
     from steward.core.alerts import _is_breached
     from steward.core.formatting import metric_label, metric_value
-    metric = rule.metric_name
+    operator = AlertOperator(operator)
     if metric in _BINARY:
         fires_on = [metric_value(metric, v) for v in (1.0, 0.0)
-                    if _is_breached(v, rule.operator, rule.threshold)]
+                    if _is_breached(v, operator, threshold)]
         if len(fires_on) == 1:
             return f"is {fires_on[0]}"
-    return f"{metric_label(metric)} {rule.operator.value} {metric_value(metric, rule.threshold)}"
+    return f"{metric_label(metric)} {operator.value} {metric_value(metric, threshold)}"
 
 
 def alert_sentence(rule: AlertRule, label: str, value: float | None,