feat: 每用户最大连接数改为可配置，默认 30

将 maxConnsPerUser 从硬编码常量(3)改为数据库动态配置项，管理员可在
/admin/vpn 隧道设置中调整，保存后对新连接立即生效。

- model/vpn.go: VpnSetting 新增 MaxConnsPerUser 字段，gorm default:30
- db/db.go: 种子数据设默认 30；旧库回填 0 值为 30
- vpn/tunnel.go: 删除 maxConnsPerUser 常量，改读 VPN.Settings()，兜底 30
- handler/vpn.go: API 响应/请求结构体新增字段，校验范围 1-1000
- VpnView.vue: 隧道设置表单新增"每用户最大连接数"输入框
- zh.ts/en.ts: 新增 maxConnsPerUser 文案，更新首页多设备描述
- docs/client-development.md: 更新常量表为可配置项

internal/handler/vpn.go

@@ -22,6 +22,7 @@ type vpnSettingsResponse struct {
 	AllowClientToClient bool   `json:"allow_client_to_client"`
 	DoLocalIPConfig     bool   `json:"do_local_ip_config"`
 	DoRemoteIPConfig    bool   `json:"do_remote_ip_config"`
+	MaxConnsPerUser     int    `json:"max_conns_per_user"`
 }
 
 type updateVpnSettingsRequest struct {
@@ -33,6 +34,7 @@ type updateVpnSettingsRequest struct {
 	AllowClientToClient *bool   `json:"allow_client_to_client"`
 	DoLocalIPConfig     *bool   `json:"do_local_ip_config"`
 	DoRemoteIPConfig    *bool   `json:"do_remote_ip_config"`
+	MaxConnsPerUser     *int    `json:"max_conns_per_user"`
 }
 
 func loadVpnSettings() (model.VpnSetting, error) {
@@ -131,6 +133,7 @@ func GetVpnSettings(c *gin.Context) {
 		AllowClientToClient: s.AllowClientToClient,
 		DoLocalIPConfig:     s.DoLocalIPConfig,
 		DoRemoteIPConfig:    s.DoRemoteIPConfig,
+		MaxConnsPerUser:     s.MaxConnsPerUser,
 	})
 }
 
@@ -185,6 +188,13 @@ func UpdateVpnSettings(c *gin.Context) {
 	if req.DoRemoteIPConfig != nil {
 		s.DoRemoteIPConfig = *req.DoRemoteIPConfig
 	}
+	if req.MaxConnsPerUser != nil {
+		if *req.MaxConnsPerUser < 1 || *req.MaxConnsPerUser > 1000 {
+			c.JSON(http.StatusBadRequest, gin.H{"error": "每用户最大连接数范围 1-1000"})
+			return
+		}
+		s.MaxConnsPerUser = *req.MaxConnsPerUser
+	}
 
 	if err := db.DB.Save(&s).Error; err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "保存设置失败"})