fix(backend): license service 小修 (#45-46-48)

- #45 Activate 幂等路径 Update 加错误检查，失败时返回 error 而非静默成功
- #46 Verify 加载 license 加 shop_id 过滤（防御纵深）
- #48 删除 ErrDeviceMismatch 死代码（多设备迁移后已无单设备约束）

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

backend/internal/service/license.go

@@ -21,7 +21,6 @@ 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")
 	ErrDeviceLimitExceed = errors.New("device limit reached — deactivate another device first")
 )
 
@@ -67,7 +66,9 @@ func (s *LicenseService) Activate(shopID uint64, licenseKey, deviceID, deviceNam
 	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)
+		if err := s.db.Model(&existing).Update("device_name", deviceName).Error; err != nil {
+			return nil, err
+		}
 		return &lic, nil
 	}
 
@@ -99,7 +100,7 @@ func (s *LicenseService) Verify(shopID uint64, deviceID string) (*model.License,
 		return nil, ErrLicenseNotFound
 	}
 	var lic model.License
-	if err := s.db.Where("id = ? AND is_active = 1", dev.LicenseID).First(&lic).Error; err != nil {
+	if err := s.db.Where("id = ? AND shop_id = ? AND is_active = 1", dev.LicenseID, shopID).First(&lic).Error; err != nil {
 		return nil, ErrLicenseNotFound
 	}
 	if lic.ExpiresAt != nil && time.Now().After(*lic.ExpiresAt) {