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

internal/api/server.go

@@ -46,6 +46,7 @@ func New(
 	hostSvc := &service.HostService{DB: queries, Redis: rdb, JWT: jwtSecret, Pool: pool}
 	teamSvc := &service.TeamService{DB: queries, Pool: pgPool, HostPool: pool}
 	auditSvc := &service.AuditService{DB: queries}
+	statsSvc := &service.StatsService{DB: queries, Pool: pgPool}
 
 	al := audit.New(queries)
 
@@ -62,6 +63,7 @@ func New(
 	teamH := newTeamHandler(teamSvc, al)
 	usersH := newUsersHandler(teamSvc)
 	auditH := newAuditHandler(auditSvc)
+	statsH := newStatsHandler(statsSvc)
 
 	// OpenAPI spec and docs.
 	r.Get("/openapi.yaml", serveOpenAPI)
@@ -109,6 +111,7 @@ func New(
 		r.Use(requireAPIKeyOrJWT(queries, jwtSecret))
 		r.Post("/", sandbox.Create)
 		r.Get("/", sandbox.List)
+		r.Get("/stats", statsH.GetStats)
 
 		r.Route("/{id}", func(r chi.Router) {
 			r.Get("/", sandbox.Get)