fix: error-pages graded healthy, unwired sensors listed apart, Maintenance failed reads 0 (#5780, #5781, #5782)

- #5780 "The error-pages service is graded on other routers' errors". The
  Traefik scrape reads which services an errors middleware sends to, and
  grades them healthy: their 4xx and 5xx are the codes of the errors they
  cover for, already graded on the routers that failed.
- #5781 "The Maintenance failed rules sit at no data until a host has had
  a maintenance job". Every host with an agent reads 0 until a job fails.
- #5782 "VDNT-Util01's host page lists SYSTIN at 115 °C in red". The host
  page lists sensors that counts_as_hottest leaves out (unwired board
  inputs, 0 °C or below) on one muted line, not as heat.

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

plugins/traefik/scheduler.py

@@ -26,6 +26,16 @@ def router_domains() -> dict[str, list[str]] | None:
     return _router_domains
 
 
+# The services Traefik's errors middlewares send error pages to (#5780), as
+# last read from its API. Kept through an API blip: that config rarely moves,
+# and dropping it would grade the error pages as failing for that while.
+_error_page_services: frozenset[str] = frozenset()
+
+
+def error_page_services() -> frozenset[str]:
+    return _error_page_services
+
+
 # Access log state
 _geoip_db = None
 _access_log_last_tick: "datetime | None" = None
@@ -48,7 +58,7 @@ def make_scrape_task(app: "Quart") -> ScheduledTask:
 
 async def _do_scrape(app: "Quart") -> None:
     """Fetch Traefik metrics, write to DB, and emit plugin_metrics."""
-    global _prev_metrics, _prev_time, _router_domains
+    global _prev_metrics, _prev_time, _router_domains, _error_page_services
 
     from datetime import datetime, timezone
     from .models import TraefikCert, TraefikGlobalStat, TraefikMetric, TraefikStatusCode
@@ -59,6 +69,7 @@ async def _do_scrape(app: "Quart") -> None:
         compute_status_codes,
         extract_certs,
         fetch_metrics,
+        fetch_error_page_services,
         fetch_router_domains,
     )
     from steward.core.alerts import MetricBatch
@@ -77,6 +88,10 @@ async def _do_scrape(app: "Quart") -> None:
         if _router_domains is not None:
             logger.warning("Traefik API stopped answering (%s); certificates shown without routers", exc)
         _router_domains = None
+    try:
+        _error_page_services = await fetch_error_page_services(api_base(metrics_url, cfg.get("api_url") or ""))
+    except Exception as exc:
+        logger.debug("Traefik middlewares not read (%s); keeping the last error-page services", exc)
 
     now = time.monotonic()
     elapsed = now - _prev_time if _prev_time else 60.0