feat(traefik): Routers widget shows current routers only, one header row, certs on the routers they serve (#5254)

#5254 "Traefik — Routers: current routers only, header row, certs on the router they serve".

- Router hosts come from Traefik's API (/api/http|tcp/routers), read
  best-effort on each scrape. An optional api_url overrides it; the default
  is /api on the metrics host. A certificate's expiry then shows in a TLS
  column on the router whose host it covers. An expiring one makes that
  router a problem, graded by Settings → Thresholds.
- Only current: routers and certificates must have appeared in the last
  three scrapes. Certificates are upserted by serial and never deleted, so
  renewed or retired ones lingered on the widget.
- Widget order: verdict, problem routers with the reason, then the busiest
  healthy routers under one header (Router / Req/s / Bandwidth / p95 / TLS).
  A missing value shows "—".
- If the API is unreachable, expiring certificates are listed after the
  routers, not above them.
- The page splits certificates into served (with the routers using each,
  plus SANs) and "No longer served".
- Latest row per router: an index walk (recursive CTE + LATERAL) replaces
  DISTINCT plus a query per router.

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

plugins/traefik/plugin.yaml

@@ -14,6 +14,9 @@ tags:
 
 config:
   metrics_url: "http://localhost:8080/metrics"
+  # Traefik's API, for which hosts each router serves (so a certificate's
+  # expiry shows on its router). Empty: /api on the metrics URL's host.
+  api_url: ""
   scrape_interval_seconds: 60
   access_log:
     enabled: false