fix(auth): 包装数据库错误为 AppError 避免裸 500

- repo.Create 错误包装为 CREATE_USER_FAILED
- FindByEmail 非 RecordNotFound 错误包装为 QUERY_USER_FAILED
- 修复无效 UUID tenant 导致 gorm 原生错误泄露为 500 的问题

internal/modules/auth/application/service.go

@@ -9,6 +9,7 @@ import (
 	"mengstack/internal/modules/auth/domain"
 
 	"golang.org/x/crypto/bcrypt"
+	"gorm.io/gorm"
 )
 
 type Service struct {
@@ -35,6 +36,8 @@ func (s *Service) Register(ctx context.Context, req domain.RegisterRequest) (dom
 
 	if _, err := s.repo.FindByEmail(ctx, tenantID, req.Email); err == nil {
 		return domain.TokenPair{}, errors.ErrUserExists
+	} else if err != gorm.ErrRecordNotFound {
+		return domain.TokenPair{}, errors.Wrap(err, "QUERY_USER_FAILED", "failed to check existing user", 500)
 	}
 
 	hash, err := bcrypt.GenerateFromPassword([]byte(req.Password), 12)
@@ -52,7 +55,7 @@ func (s *Service) Register(ctx context.Context, req domain.RegisterRequest) (dom
 	}
 
 	if err := s.repo.Create(ctx, user); err != nil {
-		return domain.TokenPair{}, err
+		return domain.TokenPair{}, errors.Wrap(err, "CREATE_USER_FAILED", "failed to create user", 500)
 	}
 
 	return s.generateTokens(user)