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/db/db.go

@@ -105,21 +105,30 @@ func seedDefaultAdmin(cfg *config.DatabaseConfig) error {
 func seedDefaultVpnSettings() error {
 	var s model.VpnSetting
 	if err := DB.First(&s, model.VpnSettingSingletonID).Error; err == nil {
+		needSave := false
 		if s.Subnet6 == "" {
 			s.Subnet6 = "fd00:dead:beef::/112"
+			needSave = true
+		}
+		if s.MaxConnsPerUser == 0 {
+			s.MaxConnsPerUser = 30
+			needSave = true
+		}
+		if needSave {
 			DB.Save(&s)
 		}
 		return nil
 	}
 	s = model.VpnSetting{
-		ID:               model.VpnSettingSingletonID,
-		Enabled:          false,
-		Subnet:           "192.168.77.0/24",
-		Subnet6:          "fd00:dead:beef::/112",
-		MTU:              1420,
-		InterfaceName:    "",
-		DoLocalIPConfig:  true,
-		DoRemoteIPConfig: true,
+		ID:                model.VpnSettingSingletonID,
+		Enabled:           false,
+		Subnet:            "192.168.77.0/24",
+		Subnet6:           "fd00:dead:beef::/112",
+		MTU:               1420,
+		InterfaceName:     "",
+		DoLocalIPConfig:   true,
+		DoRemoteIPConfig:  true,
+		MaxConnsPerUser:   30,
 	}
 	return DB.Create(&s).Error
 }

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"})

internal/model/vpn.go

@@ -14,6 +14,7 @@ type VpnSetting struct {
 	AllowClientToClient bool   `gorm:"default:false"`
 	DoLocalIPConfig     bool   `gorm:"default:true"`
 	DoRemoteIPConfig    bool   `gorm:"default:true"`
+	MaxConnsPerUser     int    `gorm:"default:30"`
 	UpdatedAt           time.Time
 }
 

internal/vpn/tunnel.go

@@ -21,8 +21,7 @@ const (
 	writeTimeout    = 10 * time.Second
 	readyTimeout    = 30 * time.Second
 	pingPeriod      = 30 * time.Second
-	maxMessageSize  = 1 << 20
-	maxConnsPerUser = 3
+	maxMessageSize = 1 << 20
 )
 
 var (
@@ -99,7 +98,11 @@ func runTunnel(conn *websocket.Conn, user *model.User) {
 	}
 
 	activeConnsMu.Lock()
-	if activeConns[user.ID] >= maxConnsPerUser {
+	maxConns := VPN.Settings().MaxConnsPerUser
+	if maxConns <= 0 {
+		maxConns = 30
+	}
+	if activeConns[user.ID] >= maxConns {
 		activeConnsMu.Unlock()
 		_ = sendJSON(conn, controlMessage{Type: "error", Message: "连接数已达上限"})
 		return