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/reports.py

@@ -7,20 +7,19 @@ from __future__ import annotations
 import asyncio
 import logging
 from datetime import datetime, timedelta, timezone
-from email.message import EmailMessage
 
 from sqlalchemy import and_, case, func, select
 
 from steward.alerts import service as alert_svc
+from steward.core.formatting import zone
+from steward.core.mail import message, report_email
+from steward.core.settings import get_setting
 from steward.models.alerts import AlertEvent
 from steward.models.metrics import PluginMetric
 from steward.models.monitors import Monitor, MonitorResult
 
 logger = logging.getLogger(__name__)
 
-_DAYS = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]
-
-
 async def build_report_data(app) -> dict:
     """Collect all data for the weekly digest."""
     now = datetime.now(timezone.utc)
@@ -55,12 +54,10 @@ async def build_report_data(app) -> dict:
         active_rows = await alert_svc.active_alerts(db)
         active_alerts = [
             {
-                "state": alert_svc.alert_label(state).upper(),
+                "state": alert_svc.alert_label(state),
                 "name": rule.name,
                 "resource": rule.resource_name,
-                "metric": rule.metric_name,
-                "operator": rule.operator.value,
-                "threshold": rule.threshold,
+                "condition": alert_svc.rule_condition(rule),
             }
             for state, rule in active_rows
         ]
@@ -120,66 +117,6 @@ async def build_report_data(app) -> dict:
     }
 
 
-def _render_report_text(data: dict) -> str:
-    now = data["generated_at"]
-    lines: list[str] = []
-
-    lines.append("Steward — Weekly Report")
-    lines.append(f"Generated: {now.strftime('%Y-%m-%d %H:%M UTC')}")
-    lines.append("")
-
-    # ── Hosts currently down ──────────────────────────────────────────────────
-    down = data["hosts_down"]
-    if down:
-        lines.append(f"⚠  {len(down)} monitor(s) currently DOWN: {', '.join(down)}")
-        lines.append("")
-
-    # ── Uptime summary ────────────────────────────────────────────────────────
-    lines.append("UPTIME SUMMARY (7-day)")
-    lines.append("─" * 40)
-    for entry in data["uptime_summary"]:
-        pct = entry["pct_7d"]
-        pct_str = f"{pct:.2f}%" if pct is not None else "no data"
-        lines.append(f"  {entry['name']:<32} {pct_str}")
-    if not data["uptime_summary"]:
-        lines.append("  (no monitors configured)")
-    lines.append("")
-
-    # ── Active alerts ─────────────────────────────────────────────────────────
-    alerts = data["active_alerts"]
-    lines.append(f"ACTIVE ALERTS ({len(alerts)})")
-    lines.append("─" * 40)
-    if alerts:
-        for a in alerts:
-            lines.append(
-                f"  [{a['state']}] {a['name']} — {a['resource']}:"
-                f"{a['metric']} {a['operator']} {a['threshold']}"
-            )
-    else:
-        lines.append("  None — all clear.")
-    lines.append("")
-
-    # ── Alert events this week ────────────────────────────────────────────────
-    ev = data["alert_events"]
-    lines.append("ALERT EVENTS THIS WEEK")
-    lines.append("─" * 40)
-    lines.append(f"  Fired:    {ev['fired']}")
-    lines.append(f"  Resolved: {ev['resolved']}")
-    lines.append("")
-
-    # ── Top Traefik routers ───────────────────────────────────────────────────
-    if data["top_routers"]:
-        lines.append("TOP TRAEFIK ROUTERS (avg req/s, last 24h)")
-        lines.append("─" * 40)
-        for r in data["top_routers"]:
-            lines.append(f"  {r['name']:<40} {r['avg_rate']:.2f} req/s")
-        lines.append("")
-
-    lines.append("─" * 40)
-    lines.append("Steward — https://git.fabledsword.com/bvandeusen/Steward")
-    return "\n".join(lines)
-
-
 async def send_report(app) -> tuple[bool, str]:
     """Build and email the weekly digest. Returns (success, message)."""
     smtp_cfg = app.config.get("SMTP", {})
@@ -192,15 +129,10 @@ async def send_report(app) -> tuple[bool, str]:
         logger.exception("Failed to build report data")
         return False, f"Data collection failed: {exc}"
 
-    body = _render_report_text(data)
-    date_str = data["generated_at"].strftime("%Y-%m-%d")
-    subject = f"[Steward] Weekly Report — {date_str}"
-
-    msg = EmailMessage()
-    msg["Subject"] = subject
-    msg["From"] = smtp_cfg.get("username", "steward@localhost")
-    msg["To"] = ", ".join(smtp_cfg["recipients"])
-    msg.set_content(body)
+    async with app.db_sessionmaker() as db:
+        tz = zone(await get_setting(db, "reports.schedule_tz"))
+    subject, text, html = report_email(smtp_cfg, data, tz)
+    msg = message(smtp_cfg, subject, text, html)
 
     try:
         loop = asyncio.get_running_loop()
@@ -231,8 +163,7 @@ def schedule_in(day: int, hour: int, saved_tz, view_tz, now: datetime | None = N
 
 async def check_and_send(app) -> None:
     """Hourly scheduler hook: send if day/hour match and not sent recently."""
-    from steward.core.formatting import zone
-    from steward.core.settings import get_setting, set_setting
+    from steward.core.settings import set_setting
 
     async with app.db_sessionmaker() as db:
         enabled = await get_setting(db, "reports.enabled")