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/http.py

@@ -154,6 +154,25 @@ async def start(
         lst = [h.jsons() for h in hosts]
         return web.json_response(json.loads("[" + ",".join(lst) + "]"))
 
+    async def api_alert_summary(request):
+        """GET /api/0/alert_summary — counts of ok/warning/critical hosts visible to caller."""
+        user, err = _require_auth(request)
+        if err:
+            return err
+        from .threshold import AlertLevel
+        critical = warning = ok = 0
+        for host in hbdclass.Host.hosts.values():
+            if not _can_operate_host(user, host):
+                continue
+            levels = {s.level for s in host.alert_states.values()}
+            if AlertLevel.CRITICAL in levels:
+                critical += 1
+            elif AlertLevel.WARNING in levels:
+                warning += 1
+            else:
+                ok += 1
+        return web.json_response({"critical": critical, "warning": warning, "ok": ok})
+
     async def api_messages(request):
         lst = data.msgs[-30:]
         return web.json_response(lst)
@@ -893,6 +912,7 @@ async def start(
             web.get("/api/0/users/{username}/avatar", api_user_avatar),
             # Hosts
             web.get("/api/0/hosts", api_hosts),
+            web.get("/api/0/alert_summary", api_alert_summary),
             web.get("/api/0/messages", api_messages),
             web.get("/api/0/hosts/{hostname}/plugins", api_host_plugins),
             web.get("/api/0/hosts/{hostname}/plugins/{plugin_name}", api_host_plugin_detail),