feat(traefik): keep each router's 4xx and 5xx responses by status code (#5711)

The scraper folded traefik_service_requests_total{code} into two shares and
dropped the code, so a problem couldn't say whether it was scanners (404), a
login wall (401/403) or the app down (502/503/504). Each scrape now stores the
responses per router and code that occurred (traefik_status_codes, migration
traefik_006), pruned with the router metrics and left out of backups (System
180 ruling: Traefik history is not backed up). A problem card lists its codes
over the last 15 minutes, most first, or says none occurred, or that codes
aren't recorded yet.

A counter series new since the previous scrape now counts from 0 instead of
being skipped: Traefik creates a router's 502 series at its first 502, so a
first burst of 5xx read as 0% until it happened again.

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

plugins/traefik/__init__.py

@@ -11,10 +11,13 @@ _app: "Quart | None" = None
 def setup(app: "Quart") -> None:
     global _app
     _app = app
-    from .models import TraefikMetric, TraefikCert, TraefikGlobalStat, TraefikAccessSummary  # noqa: F401
+    from .models import (  # noqa: F401
+        TraefikAccessSummary, TraefikCert, TraefikGlobalStat, TraefikMetric, TraefikStatusCode,
+    )
     from steward.core.cleanup import prune_by_age
     # Samples older than data.retention_days are pruned by the cleanup (#5155).
     prune_by_age(TraefikMetric.scraped_at)
+    prune_by_age(TraefikStatusCode.scraped_at)
     prune_by_age(TraefikGlobalStat.scraped_at)
     prune_by_age(TraefikAccessSummary.period_end)