feat: add public status page with component health monitoring and system metrics visualization

Add statuspage package with service, handler, and types for exposing platform health. Implement GET /api/v1/status endpoint returning operational status, component health (API, database, gateway runtime), and control plane summary counts.

Add Service.Snapshot method querying database connectivity, user/device/gateway/service/policy counts, connected device count via handshake timestamps, and gateway runtime tel

backend/internal/app/app.go

@@ -17,6 +17,7 @@ import (
 	"nexavpn/backend/internal/ipam"
 	"nexavpn/backend/internal/policy"
 	"nexavpn/backend/internal/servicecatalog"
+	"nexavpn/backend/internal/statuspage"
 	"nexavpn/backend/internal/user"
 )
 
@@ -46,6 +47,7 @@ func New(cfg config.Config) (*App, error) {
 	gatewayService := gateway.NewService(gateway.NewPGRepository(pool))
 	deviceService := device.NewService(device.NewPGRepository(pool), policyService, gatewayService, ipam.NewService())
 	auditService := audit.NewService(audit.NewPGRepository(pool))
+	statusService := statuspage.NewService(pool)
 
 	router := httpserver.NewRouter(cfg.JWTSecret, httpserver.Handlers{
 		Auth:    auth.NewHandler(authService, auditService),
@@ -56,6 +58,7 @@ func New(cfg config.Config) (*App, error) {
 		Policy:  policy.NewHandler(policyService, auditService),
 		Gateway: gateway.NewHandler(gatewayService, cfg.GatewayBootstrapToken),
 		Audit:   audit.NewHandler(auditService),
+		Status:  statuspage.NewHandler(statusService),
 	})
 
 	return &App{