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/host_agent/templates/_host_metrics.html

@@ -159,6 +159,12 @@
     </div>
     {% endfor %}
   </div>
+  {% if unwired_temps %}
+  {# Board inputs with nothing wired to them, and readings at or below 0 °C:
+     constant at any load, left out of the host's temperature (#5759). #}
+  <p class="field-hint" style="margin:0.5rem 0 0;">Not counted, reading a constant with nothing wired to them:
+    {% for label, c in unwired_temps.items() %}{{ label }} {{ "%.0f"|format(c) }}°C{{ ", " if not loop.last }}{% endfor %}.</p>
+  {% endif %}
 </div>
 {% endif %}