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/backup/contents.py

@@ -25,6 +25,9 @@ KEPT: frozenset[str] = frozenset({
     "unifi_known_clients",
     # History kept by the ruling.
     "audit_events", "alert_events",
+    # Metric anomalies and their reviews (#5740): what someone marked normal
+    # or real is a decision, kept like alert history.
+    "metric_anomalies", "metric_anomaly_spans",
 })
 
 # Rows that are measurements, logs or in-flight work: left out.