feat(norms): Steward learns each metric's normal per hour of day, then week (#5739)

A metric registered with learn_normal() gets, for every resource reporting
it, a usual hold (the 90th percentile across past days of each hour's p95)
and a usual peak for each slot:
- an hour of the day once 5 days of history exist;
- an hour of the week once history reaches back 3 weeks, so weekly jobs are
  learned too.
Each slot takes in the hours either side, so a job that runs a little late
stays normal. One SQL statement relearns everything hourly into
metric_norms, from the rollup plus the raw window not rolled up yet;
norms_at() reads the slot a moment falls in. Nothing is registered yet:
Traefik (#5741) and hosts (#5742) opt in.

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

steward/models/__init__.py

@@ -6,6 +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 .alerts import AlertRule, AlertState, AlertEvent, AlertOperator, AlertStateEnum
 from .ansible import AnsibleRun, AnsibleRunStatus
 from .ansible_inventory import AnsibleTarget, AnsibleGroup, ansible_target_groups
@@ -18,7 +19,7 @@ __all__ = [
     "Base", "User", "ApiToken", "UserInvite", "PasswordReset",
     "Host",
     "Monitor", "MonitorResult", "MonitorType",
-    "PluginMetric",
+    "PluginMetric", "MetricNorm",
     "AlertRule", "AlertState", "AlertEvent", "AlertOperator", "AlertStateEnum",
     "AnsibleRun", "AnsibleRunStatus",
     "AnsibleTarget", "AnsibleGroup", "ansible_target_groups",