feat: owner Update/Delete buttons on Host Overview; purge stale alerts on reload

Host Overview (plugins.html): show Update and Delete buttons in the
host-right zone when the logged-in user is the host owner (or admin /
unauthenticated mode). Buttons link to /u?h=<host> and /d?h=<host>
with stopPropagation so they don't toggle the accordion; Delete prompts
for confirmation first.

ThresholdChecker.purge_stale_alerts(): removes alert states whose
metric_path has no matching threshold in the current config. Called
after startup pickle restore and after every SIGHUP config reload so
alerts orphaned by upgrades or config changes do not persist
indefinitely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hbd/server/templates/plugins.html

@@ -131,6 +131,27 @@
       text-overflow: ellipsis;
     }
 
+    .host-action-btn {
+      font-size: 0.75em;
+      font-weight: bold;
+      padding: 3px 10px;
+      border-radius: 4px;
+      border: none;
+      cursor: pointer;
+      text-decoration: none;
+      white-space: nowrap;
+    }
+    .host-action-btn.update-btn {
+      background: #e3f2fd;
+      color: #1565c0;
+    }
+    .host-action-btn.update-btn:hover { background: #bbdefb; }
+    .host-action-btn.delete-btn {
+      background: #ffebee;
+      color: #c62828;
+    }
+    .host-action-btn.delete-btn:hover { background: #ffcdd2; }
+
     /* ── Host body ──────────────────────────────────────────────── */
 
     .host-body {
@@ -379,6 +400,14 @@
               <span class="nagios-badge" id="nagios-badge-{{ host.name }}">—</span>
               {% endif %}
               <span class="os-label" id="os-label-{{ host.name }}"></span>
+              {% if host.is_owner %}
+              <a class="host-action-btn update-btn"
+                 href="/u?h={{ host.name }}"
+                 onclick="event.stopPropagation()">Update</a>
+              <a class="host-action-btn delete-btn"
+                 href="/d?h={{ host.name }}"
+                 onclick="event.stopPropagation(); return confirm('Delete host {{ host.name }}?')">Delete</a>
+              {% endif %}
             </div>
           </div>