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>

steward/core/settings.py

@@ -97,6 +97,9 @@ DEFAULTS: dict[str, Any] = {
     # than this share of 4xx is worth a look (#5380).
     "traefik.threshold.error_5xx_pct": 0.1,
     "traefik.threshold.error_4xx_pct": 1.0,
+    # A router's 4xx share over fewer requests than this in 15 minutes isn't
+    # graded, on the page or for anomalies: one 404 of three is 33% (#5741).
+    "traefik.threshold.min_requests": 30,
     # Degraded/critical cutoffs for metric coloring (warn=amber, crit=red).
     # cpu/mem/disk/load are percentages (load is load-per-core %); temp in °C.
     # uptime is "higher is better" so its warn/crit are floors. Ping latency
@@ -534,6 +537,9 @@ def to_thresholds_cfg(settings: dict[str, Any]) -> dict:
                       "crit": g("traefik.threshold.error_5xx_pct", 0.1), "dir": "high"},
         "error_4xx": {"warn": g("traefik.threshold.error_4xx_pct", 1.0),
                       "crit": g("traefik.threshold.error_4xx_pct", 1.0), "dir": "high"},
+        # Fewer requests than this in 15 minutes and a router's 4xx share isn't graded.
+        "router_requests": {"warn": g("traefik.threshold.min_requests", 30),
+                            "crit": g("traefik.threshold.min_requests", 30), "dir": "low"},
     }