feat: 管理后台仪表盘统计卡片接入真实数据

新增 /api/admin/stats 聚合接口，填充运行时长、活跃设备、今日流量、在线节点四项卡片：
- VpnService 记录启动时间，提供 StartedAt/TotalLiveTraffic 方法
- 新增 TrafficStat 模型，客户端断开时 upsert 当日 rx/tx 流量（持久化）
- 前端 AdminView 调用 stats 接口并每 30 秒自动刷新

internal/handler/stats.go

@@ -0,0 +1,43 @@
+package handler
+
+import (
+	"net/http"
+	"time"
+
+	"lmvpn/internal/db"
+	"lmvpn/internal/model"
+	"lmvpn/internal/vpn"
+
+	"github.com/gin-gonic/gin"
+)
+
+type adminStatsResponse struct {
+	UptimeSeconds     int64 `json:"uptime_seconds"`
+	ActiveDevices     int   `json:"active_devices"`
+	TodayTrafficBytes int64 `json:"today_traffic_bytes"`
+	OnlineNodes       int   `json:"online_nodes"`
+}
+
+func GetAdminStats(c *gin.Context) {
+	var uptime int64
+	var onlineNodes int
+	if vpn.VPN.Running() {
+		uptime = int64(time.Since(vpn.VPN.StartedAt()).Seconds())
+		onlineNodes = 1
+	}
+
+	activeDevices := len(vpn.VPN.ClientList())
+
+	today := time.Now().Format("2006-01-02")
+	var stat model.TrafficStat
+	db.DB.Where("date = ?", today).First(&stat)
+	liveRx, liveTx := vpn.VPN.TotalLiveTraffic()
+	todayTraffic := stat.RxBytes + stat.TxBytes + liveRx + liveTx
+
+	c.JSON(http.StatusOK, adminStatsResponse{
+		UptimeSeconds:     uptime,
+		ActiveDevices:     activeDevices,
+		TodayTrafficBytes: todayTraffic,
+		OnlineNodes:       onlineNodes,
+	})
+}