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/statuspage/handler.go

@@ -0,0 +1,24 @@
+package statuspage
+
+import (
+	"net/http"
+
+	"nexavpn/backend/internal/apiutil"
+)
+
+type Handler struct {
+	service *Service
+}
+
+func NewHandler(service *Service) *Handler {
+	return &Handler{service: service}
+}
+
+func (h *Handler) PublicStatus(w http.ResponseWriter, r *http.Request) {
+	snapshot := h.service.Snapshot(r.Context())
+	statusCode := http.StatusOK
+	if snapshot.Status != "operational" {
+		statusCode = http.StatusServiceUnavailable
+	}
+	apiutil.JSON(w, statusCode, snapshot)
+}