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/settings/thresholds.html

@@ -91,6 +91,18 @@
 </div>
 {% endmacro %}
 
+<div class="card" style="max-width:640px;margin-top:1.5rem;">
+  <h2 class="section-title" style="margin-bottom:0.5rem;">Metric anomalies</h2>
+  <p style="font-size:0.82rem;color:var(--text-muted);margin-bottom:1.25rem;">
+    A metric that leaves what is normal for it at that hour is listed under
+    Alerts → Anomalies to review. One nobody marks normal or real within this
+    window, once it has ended, lapses: it leaves the list and is kept as not
+    reviewed, and its hours are never learned.
+  </p>
+  {{ days("Review window", "anomalies_review_days", "anomalies.review_days",
+          "An anomaly still going never lapses.") }}
+</div>
+
 <div class="card" style="max-width:640px;margin-top:1.5rem;">
   <h2 class="section-title" style="margin-bottom:0.5rem;">Docker data retention</h2>
   <p style="font-size:0.82rem;color:var(--text-muted);margin-bottom:1.25rem;">