fix(traefik): a router's 5xx share is judged only over enough requests, like its 4xx share (#5859)

Two 500s of about sixteen requests graded roundtable@swarm critical at
9.1%. The Settings → Thresholds minimum of requests in 15 minutes (30 by
default) now guards both shares; the 5xx share still keeps its fixed
cutoff whatever the router's history.

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

docs/architecture.md

@@ -123,7 +123,7 @@ What learns a normal, and the floor each departure must clear (Settings → Thre
 | Monitor response time | `<type>` / `response_ms` | 100 ms |
 | Router 4xx share, p95 | `traefik` / `error_rate_4xx_pct`, `latency_p95_ms` | 2 points, 50 ms |
 
-Host metrics learn with `parts=False`: the host learns, its cores (`<host>:core<N>`) don't. Disk space, a monitor that is down, an agent gone quiet and a router's 5xx share are failures and keep their fixed cutoffs.
+Host metrics learn with `parts=False`: the host learns, its cores (`<host>:core<N>`) don't. Disk space, a monitor that is down, an agent gone quiet and a router's 5xx share are failures and keep their fixed cutoffs (a router's shares are judged only over enough requests).
 
 Pages grade by the normal through `anomalies.graded()` (hosts' `host_grades` and `worst_reading(normals=)`, `docker.service.usage(thresholds=, norms=)`, `monitors.service.latency_grade`): a reading past its cutoff but within its normal reads healthy and says what is usual at this hour. The host, monitor, container and Traefik pages list their anomalies with `_anomaly_card.html`.