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.

backend/app/api/v1/router.py

@@ -1,6 +1,6 @@
 from fastapi import APIRouter
 
-from app.api.v1 import auth, events, health, hosts, problems
+from app.api.v1 import auth, dashboards, events, health, hosts, problems
 
 api_router = APIRouter()
 api_router.include_router(health.router)
@@ -8,3 +8,4 @@ api_router.include_router(auth.router)
 api_router.include_router(events.router)
 api_router.include_router(hosts.router)
 api_router.include_router(problems.router)
+api_router.include_router(dashboards.router)