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/core/notifications.py

@@ -41,27 +41,11 @@ def smtp_mode(port, tls) -> str:
     return "starttls" if tls else "plain"
 
 
-def email_body(alert: dict) -> str:
-    value = "no data" if alert.get("value") is None else alert["value"]
-    return (
-        f"Alert: {alert['state']}\n"
-        f"Rule: {alert['rule_name']}\n"
-        f"Resource: {alert['resource']}\n"
-        f"Metric: {alert['metric']} = {value}\n"
-        f"Threshold: {alert['threshold']}\n"
-        + (f"Note: {alert['note']}\n" if alert.get("note") else "")
-        + f"Time: {alert['timestamp']}\n"
-    )
-
-
 async def _send_email(cfg: dict, alert: dict) -> dict:
+    from steward.core.mail import alert_email, message
     try:
-        msg = EmailMessage()
-        # A header may not hold a line break; a rule name can (MCP takes any string).
-        msg["Subject"] = " ".join(f"[Steward] {alert['state']} — {alert['rule_name']}".split())
-        msg["From"] = cfg.get("from") or cfg.get("username") or "steward@localhost"
-        msg["To"] = ", ".join(cfg["recipients"])
-        msg.set_content(email_body(alert))
+        subject, text, html = alert_email(cfg, alert)
+        msg = message(cfg, subject, text, html)
         loop = asyncio.get_running_loop()
         await loop.run_in_executor(None, _smtp_send_sync, cfg, msg)
         return {"sent": True, "error": None}