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/models/__init__.py

@@ -6,7 +6,7 @@ from .password_resets import PasswordReset
 from .hosts import Host
 from .monitors import Monitor, MonitorResult, MonitorType
 from .metrics import PluginMetric
-from .norms import MetricNorm
+from .norms import MetricNorm, MetricAnomaly, MetricAnomalySpan
 from .alerts import AlertRule, AlertState, AlertEvent, AlertOperator, AlertStateEnum
 from .ansible import AnsibleRun, AnsibleRunStatus
 from .ansible_inventory import AnsibleTarget, AnsibleGroup, ansible_target_groups
@@ -19,7 +19,7 @@ __all__ = [
     "Base", "User", "ApiToken", "UserInvite", "PasswordReset",
     "Host",
     "Monitor", "MonitorResult", "MonitorType",
-    "PluginMetric", "MetricNorm",
+    "PluginMetric", "MetricNorm", "MetricAnomaly", "MetricAnomalySpan",
     "AlertRule", "AlertState", "AlertEvent", "AlertOperator", "AlertStateEnum",
     "AnsibleRun", "AnsibleRunStatus",
     "AnsibleTarget", "AnsibleGroup", "ansible_target_groups",