feat(promo-code): complete promo code feature implementation

- Add promo_code_enabled field to SystemSettings and PublicSettings DTOs
- Add promo code validation in registration flow
- Add admin settings UI for promo code configuration
- Add i18n translations for promo code feature

backend/internal/service/auth_service.go

@@ -153,8 +153,8 @@ func (s *AuthService) RegisterWithVerification(ctx context.Context, email, passw
 		return "", nil, ErrServiceUnavailable
 	}
 
-	// 应用优惠码（如果提供）
-	if promoCode != "" && s.promoService != nil {
+	// 应用优惠码（如果提供且功能已启用）
+	if promoCode != "" && s.promoService != nil && s.settingService != nil && s.settingService.IsPromoCodeEnabled(ctx) {
 		if err := s.promoService.ApplyPromoCode(ctx, user.ID, promoCode); err != nil {
 			// 优惠码应用失败不影响注册，只记录日志
 			log.Printf("[Auth] Failed to apply promo code for user %d: %v", user.ID, err)