feat(traefik): routers are graded against their own normal (#5741)

#5741 "Traefik routers are graded against their own normal", milestone 513.

- A router's 4xx share and p95 learn their normal (learn_normal in the
  plugin's setup). Once a router has 5 days of history, a reading over the
  fixed cutoff but inside its normal is healthy, and the healthy table says
  it is usual for that router. One out of its normal is a warning that says
  what is usual, and a metric anomaly.
- The 5xx share is a failure: fixed cutoff always, whatever the history or
  traffic (System 182's ruling).
- A 4xx share over fewer requests than Settings -> Thresholds' new "Fewest
  requests to judge 4xx" (30 per 15 minutes) isn't graded on the page, and
  isn't judged for anomalies.
- Core: learn_normal(gate=(metric, least)) judges a key only while its
  resource's gate metric averages at least `least`; `least` may be a
  function, so it follows a setting. learn_normal(since=...) learns only
  from history after a moment: Traefik latency learns from #5755's
  per-scrape figures only.
- The Traefik page lists the routers' metric anomalies of the last 30 days
  (_anomaly_card.html). The dashboard widget keeps its why to problems.
- Thresholds and docs say when the cutoffs give way to a normal.

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

docs/plugins/traefik.md

@@ -89,6 +89,8 @@ The full Traefik page at `/plugins/traefik/` reads top to bottom, for the time r
 
 A router is graded on its last 15 minutes against Settings → Thresholds. When a problem started is read from the router's history over the range.
 
+Once a router has 5 days of history, its 4xx share and p95 latency are judged against its own normal for that hour (see [Normals and anomalies](../architecture.md)) instead of the fixed cutoffs. One over the cutoff but within its normal is healthy, and the healthy table says it is usual for that router (an error-pages service that answers 4xx all day, a login wall's steady 401s). One out of its normal is a warning that says what is usual, and a metric anomaly to review; the Traefik page lists the routers' anomalies of the last 30 days. A 4xx share over fewer requests in the 15 minutes than Settings → Thresholds allows (30 by default) isn't judged at all. The 5xx share is a failure and always keeps its fixed cutoff. Latency is learned only from #5755's per-scrape figures, so its normal arrives 5 days after that release.
+
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