Add live stats page with metrics sampling and route split

- New sandbox_metrics_snapshots table sampled every 10s (60-day retention)
- Background MetricsSampler goroutine wired into control plane startup
- GET /v1/sandboxes/stats?range=5m|1h|6h|24h|30d endpoint with adaptive
  polling intervals; reserved CPU/RAM uses ceil(paused/2) formula
- StatsPanel component: 4 stat cards + 2 Chart.js line charts (straight
  lines, integer y-axis for running count, dual-axis for CPU/RAM)
- Range filter persisted in URL query param; polls update data silently
  (no blink — loading state only shown on initial mount)
- Split /dashboard/capsules into /list and /stats sub-routes with shared
  layout; capsuleRunningCount store syncs badge across routes
- CreateCapsuleDialog extracted as reusable component

cmd/control-plane/main.go

@@ -94,6 +94,10 @@ func main() {
 	monitor := api.NewHostMonitor(queries, hostPool, audit.New(queries), 30*time.Second)
 	monitor.Start(ctx)
 
+	// Start metrics sampler (records per-team sandbox stats every 10s).
+	sampler := api.NewMetricsSampler(queries, 10*time.Second)
+	sampler.Start(ctx)
+
 	httpServer := &http.Server{
 		Addr:    cfg.ListenAddr,
 		Handler: srv.Handler(),