feat: Dashboard MVP with summary API and UI

Add GET /api/v1/dashboards/summary (24h events, open problems,
hosts, severity breakdown, recent events) and Vue /dashboard page.

frontend/src/router.ts

@@ -5,11 +5,13 @@ import EventDetailView from "./views/EventDetailView.vue";
 import HostsView from "./views/HostsView.vue";
 import LoginView from "./views/LoginView.vue";
 import ProblemsView from "./views/ProblemsView.vue";
+import DashboardView from "./views/DashboardView.vue";
 
 const router = createRouter({
   history: createWebHistory(),
   routes: [
-    { path: "/", redirect: "/events" },
+    { path: "/", redirect: "/dashboard" },
+    { path: "/dashboard", component: DashboardView },
     { path: "/login", component: LoginView },
     { path: "/events", component: EventsView },
     { path: "/events/:id", component: EventDetailView, props: true },