添加 WebSocket VPN 接口及用户认证

- 移动 config 包到 internal/config，新增 DatabaseConfig
- 使用 GORM + SQLite 管理用户数据，预留 MySQL 支持
- 新增 internal/model/user.go GORM 用户模型
- 新增 internal/db/db.go 数据库初始化及默认管理员
- 新增 internal/vpn/ WebSocket 鉴权与隧道骨架
- /ws 路径支持用户名+密码鉴权 (bcrypt)
- 心跳保活 + echo 隧道模式

internal/vpn/tunnel.go

@@ -0,0 +1,53 @@
+package vpn
+
+import (
+	"log"
+	"time"
+
+	"lmvpn/internal/model"
+
+	"github.com/gorilla/websocket"
+)
+
+const (
+	readTimeout  = 60 * time.Second
+	writeTimeout = 10 * time.Second
+	pingPeriod   = 30 * time.Second
+)
+
+func runTunnel(conn *websocket.Conn, user *model.User) {
+	defer conn.Close()
+
+	log.Printf("用户 %s 已连接", user.Username)
+
+	go func() {
+		ticker := time.NewTicker(pingPeriod)
+		defer ticker.Stop()
+		for range ticker.C {
+			conn.SetWriteDeadline(time.Now().Add(writeTimeout))
+			if err := conn.WriteMessage(websocket.PingMessage, nil); err != nil {
+				return
+			}
+		}
+	}()
+
+	conn.SetPongHandler(func(string) error {
+		conn.SetReadDeadline(time.Now().Add(readTimeout))
+		return nil
+	})
+
+	for {
+		conn.SetReadDeadline(time.Now().Add(readTimeout))
+		messageType, data, err := conn.ReadMessage()
+		if err != nil {
+			log.Printf("用户 %s 断开连接: %v", user.Username, err)
+			return
+		}
+
+		conn.SetWriteDeadline(time.Now().Add(writeTimeout))
+		if err := conn.WriteMessage(messageType, data); err != nil {
+			log.Printf("用户 %s 发送失败: %v", user.Username, err)
+			return
+		}
+	}
+}