fix(backend): 架构质量改进批次二 (#32-37)

- #32 License 激活迁移到 license_devices 表：Activate/Verify/Deactivate 全部改用
  license_devices，新增 max_devices 校验和 GET /license/devices 端点；
  Activate 现在校验 shop_id 防跨租户激活
- #33 checkInventory 从 StockOutHandler 移到 StockService.CheckInventoryAvailability
- #34 新增 util/response.go 统一错误响应工具（RespondError/RespondSuccess/RespondCreated）
- #35 生产模式 CORS Origin='*' 启动时 Fatal
- #36 生产模式 License 私钥未配置启动时 Fatal
- #37 新增 util/page.go ValidatePageSize，应用到 partner/product/stock_in/stock_out handler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/service/license.go

@@ -18,10 +18,11 @@ import (
 )
 
 var (
-	ErrLicenseNotFound = errors.New("license not found")
-	ErrLicenseInactive = errors.New("license is inactive")
-	ErrLicenseExpired  = errors.New("license has expired")
-	ErrDeviceMismatch  = errors.New("license is bound to another device")
+	ErrLicenseNotFound   = errors.New("license not found")
+	ErrLicenseInactive   = errors.New("license is inactive")
+	ErrLicenseExpired    = errors.New("license has expired")
+	ErrDeviceMismatch    = errors.New("license is bound to another device")
+	ErrDeviceLimitExceed = errors.New("device limit reached — deactivate another device first")
 )
 
 type LicenseService struct {
@@ -47,10 +48,12 @@ func GenerateKey(shopID uint64, licenseType string, expiresAt *time.Time) string
 	return fmt.Sprintf("%s-%s-%s-%s", raw[0:5], raw[5:10], raw[10:15], raw[15:20])
 }
 
-// Activate 激活许可证（绑定设备）
-func (s *LicenseService) Activate(licenseKey, deviceID string) (*model.License, error) {
+// Activate 激活许可证并绑定设备到 license_devices 表。
+// 若该设备已绑定，则更新 last_seen_at（幂等）。
+// 若是新设备，则校验是否超出 max_devices 上限。
+func (s *LicenseService) Activate(shopID uint64, licenseKey, deviceID, deviceName, platform string) (*model.License, error) {
 	var lic model.License
-	if err := s.db.Where("license_key = ?", licenseKey).First(&lic).Error; err != nil {
+	if err := s.db.Where("license_key = ? AND shop_id = ?", licenseKey, shopID).First(&lic).Error; err != nil {
 		return nil, ErrLicenseNotFound
 	}
 	if !lic.IsActive {
@@ -59,23 +62,44 @@ func (s *LicenseService) Activate(licenseKey, deviceID string) (*model.License,
 	if lic.ExpiresAt != nil && time.Now().After(*lic.ExpiresAt) {
 		return nil, ErrLicenseExpired
 	}
-	// 若已绑定设备，校验是否一致
-	if lic.DeviceID != "" && lic.DeviceID != deviceID {
-		return nil, ErrDeviceMismatch
+
+	var existing model.LicenseDevice
+	err := s.db.Where("license_id = ? AND device_id = ?", lic.ID, deviceID).First(&existing).Error
+	if err == nil {
+		// Device already bound — just touch last_seen_at (handled by autoUpdateTime)
+		s.db.Model(&existing).Update("device_name", deviceName)
+		return &lic, nil
 	}
 
-	now := time.Now()
-	lic.DeviceID = deviceID
-	lic.ActivatedAt = &now
-	s.db.Save(&lic)
+	// New device — enforce max_devices
+	var count int64
+	s.db.Model(&model.LicenseDevice{}).Where("license_id = ?", lic.ID).Count(&count)
+	if int(count) >= lic.MaxDevices {
+		return nil, ErrDeviceLimitExceed
+	}
+
+	dev := model.LicenseDevice{
+		LicenseID:  lic.ID,
+		ShopID:     shopID,
+		DeviceID:   deviceID,
+		DeviceName: deviceName,
+		Platform:   platform,
+	}
+	if err := s.db.Create(&dev).Error; err != nil {
+		return nil, err
+	}
 	return &lic, nil
 }
 
-// Verify 验证（客户端启动时调用）
+// Verify 验证设备许可证（客户端启动时调用）。
+// 通过 license_devices 表查找设备，再加载对应的许可证做有效性检查。
 func (s *LicenseService) Verify(shopID uint64, deviceID string) (*model.License, error) {
+	var dev model.LicenseDevice
+	if err := s.db.Where("shop_id = ? AND device_id = ?", shopID, deviceID).First(&dev).Error; err != nil {
+		return nil, ErrLicenseNotFound
+	}
 	var lic model.License
-	if err := s.db.Where("shop_id = ? AND device_id = ? AND is_active = 1", shopID, deviceID).
-		First(&lic).Error; err != nil {
+	if err := s.db.Where("id = ? AND is_active = 1", dev.LicenseID).First(&lic).Error; err != nil {
 		return nil, ErrLicenseNotFound
 	}
 	if lic.ExpiresAt != nil && time.Now().After(*lic.ExpiresAt) {
@@ -94,11 +118,19 @@ func (s *LicenseService) ShopInfo(shopID uint64) (*model.License, error) {
 	return &lic, nil
 }
 
-// Deactivate 解绑设备（换机时使用）
+// ListDevices 列出许可证下所有已绑定设备。
+func (s *LicenseService) ListDevices(shopID uint64) ([]model.LicenseDevice, error) {
+	var devs []model.LicenseDevice
+	if err := s.db.Where("shop_id = ?", shopID).Order("activated_at DESC").Find(&devs).Error; err != nil {
+		return nil, err
+	}
+	return devs, nil
+}
+
+// Deactivate 解绑设备（从 license_devices 删除该条记录）。
 func (s *LicenseService) Deactivate(shopID uint64, deviceID string) error {
-	return s.db.Model(&model.License{}).
-		Where("shop_id = ? AND device_id = ?", shopID, deviceID).
-		Updates(map[string]interface{}{"device_id": "", "activated_at": nil}).Error
+	return s.db.Where("shop_id = ? AND device_id = ?", shopID, deviceID).
+		Delete(&model.LicenseDevice{}).Error
 }
 
 // createTrialLicense 在注册事务中为新门店签发 30 天 trial license。