fix(maintenance): a late return, once back, stays back, and every reader agrees (#6175)

18032a4 re-checked a "not back in service" job every minute with no end,
so any later swarm wobble on the host (radarr's failed task during the
downloads move, say) flipped maintenance_failed back to 1: five false
firings and ten emails for Docker02 overnight on 10-19. And the overview's
finding never learned the rule at all, so it still said Docker02's
maintenance failed three days on.

The job now keeps came_back_at, set the first time its host is found back.
maintenance.still_failed(job) is the one answer: the figure, the overview,
the cadence page's run rows and the host panel all ask it. A failed update
or a timed-out job still flags its host until a job succeeds.

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

plugins/host_agent/templates/panel.html

@@ -26,9 +26,9 @@
     {# Steward's own maintenance job on this host (#5767): a running one first. #}
     {% if maint_job %}
     {% set running = maint_job.state not in ('done', 'failed', 'timed_out') %}
-    <div class="{{ 'tone-warn' if running or maint_job.state != 'done' else '' }}" title="{{ maint_job.detail or '' }}">
+    <div class="{{ 'tone-warn' if running or maint_failed else '' }}" title="{{ maint_job.detail or '' }}">
       <a href="/plugins/host_agent/maintenance" style="color:inherit;">Maintenance</a>: {{ maint_summary }}
-      <span style="color:var(--text-muted);">· {% if running %}since {{ maint_job.state_at|ago }}{% else %}{{ maint_job.finished_at|ago }}{% endif %}{% if maint_job.state in ('failed', 'timed_out') and maint_job.detail %} · {{ maint_job.detail|truncate(90) }}{% endif %}</span>
+      <span style="color:var(--text-muted);">· {% if running %}since {{ maint_job.state_at|ago }}{% else %}{{ maint_job.finished_at|ago }}{% endif %}{% if maint_failed and maint_job.detail %} · {{ maint_job.detail|truncate(90) }}{% endif %}</span>
     </div>
     {% endif %}
     {% if reg.reboot_required %}