feat: add alert pie chart to nav bar

Show a colour-coded pie chart (red=critical, yellow=warning, green=ok)
to the left of the clock in the nav bar. Backed by a new
GET /api/0/alert_summary endpoint that counts hosts per alert level
for the current user's visible hosts.

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

hbd/server/templates/head.html

@@ -126,11 +126,17 @@
       }
 
       /* Swiss railway clock — nav */
-      .nav-clock {
+      .nav-pie {
         flex-shrink: 0;
         line-height: 0;
         margin-left: auto;
         padding: 4px 4px 4px 0;
+      }
+      #alert-pie { display: block; cursor: default; }
+      .nav-clock {
+        flex-shrink: 0;
+        line-height: 0;
+        padding: 4px 4px 4px 0;
         cursor: pointer;
       }
       #swiss-clock { display: block; }