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>

tests/integration/test_host_maintenance_cadence.py

@@ -107,7 +107,9 @@ def test_hosts_go_one_at_a_time_and_a_failure_stops_the_run():
             async with s.begin():
                 plan = await s.get(models.HostMaintenancePlan, seed["plan_id"])
                 await cadence.start_run(s, plan, datetime.now(timezone.utc), manual=True)
-        await _tick(app)
+        first = await _tick(app)
+        # Before any job finishes, every agent host reads 0 rather than nothing.
+        assert first[seed["hosts"]["a"].name] == 0.0 and first[seed["hosts"]["b"].name] == 0.0
 
         # Only the first host (by name) has a job; the second waits.
         ta, tb = seed["tokens"]["a"], seed["tokens"]["b"]