feat(anomalies): a metric out of its normal is an anomaly to review (#5740)

#5740 "A metric that departs from its normal is recorded as an anomaly to
review, and is findable after it ends", milestone 513.

- Every minute each learned key's last 15 minutes is judged against its
  hour's normal: above the usual hold x1.25 and by the metric's floor opens
  an occurrence, which ends when the level comes back.
- metric_anomalies holds one row per item to review; a repeat while it
  still waits joins it as an occurrence (metric_anomaly_spans).
- An ended row nobody reviews within anomalies.review_days (14, Settings ->
  Thresholds) lapses; an ongoing one never does.
- Learning skips every hour an occurrence covers unless its anomaly was
  marked normal: no normal is learned without review.
- Alerts -> Anomalies lists those to review first, then the reviewed and
  lapsed under day dividers; marking is operator/admin and audited.
- The Overview widget and get_overview warn of open ones and count those
  to review; the host page shows its own; the weekly report lists the
  week's and the lapsed; MCP list_anomalies and review_anomaly.
- Reviewed and lapsed rows are pruned at data.retention_days, and kept
  in backups like alert history.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/templates/email/report.txt

@@ -21,6 +21,20 @@ UPTIME OVER 7 DAYS{% if base %} ({{ base }}/monitors/){% endif %}
 {% else %}
 No monitors are enabled.
 {% endfor %}
+{% if anomalies.seen or anomalies.lapsed %}
+
+METRIC ANOMALIES THIS WEEK{% if base %} ({{ base }}/alerts/anomalies){% endif %}
+
+{% for a in anomalies.seen %}
+- {{ a.review }}: {{ a.name }}, {{ a.when }} ({{ a.level }})
+{% endfor %}
+{% for a in anomalies.lapsed %}
+- {{ a.review }}: {{ a.name }}, lapsed unreviewed; {{ a.when }} ({{ a.level }})
+{% endfor %}
+{% if anomalies.to_review %}
+{{ anomalies.to_review }} {{ "anomaly waits" if anomalies.to_review == 1 else "anomalies wait" }} to be marked normal or real.
+{% endif %}
+{% endif %}
 {% if d.top_routers %}
 
 BUSIEST TRAEFIK ROUTERS, LAST 24 HOURS{% if base %} ({{ base }}/plugins/traefik/){% endif %}