fix(backend): resolve signup issues by fixing tenant slug case-sensitivity and exposing Kratos errors

backend/internal/handler/auth_handler.go

@@ -473,6 +473,8 @@ func (h *AuthHandler) Signup(c *fiber.Ctx) error {
 		normalizedPhone = "+" + normalizedPhone
 	}
 
+	slog.Info("[Signup] Phone normalization", "raw", req.Phone, "normalized", normalizedPhone)
+
 	// IDP에 전달할 BrokerUser 스키마 구성
 	attributes := map[string]interface{}{
 		"department":      req.Department,
@@ -517,7 +519,8 @@ func (h *AuthHandler) Signup(c *fiber.Ctx) error {
 		if strings.Contains(err.Error(), "already exists") {
 			return errorJSON(c, fiber.StatusConflict, "User already exists")
 		}
-		return errorJSON(c, fiber.StatusInternalServerError, "Failed to create user")
+		// Include the actual error message in the response for debugging
+		return errorJSON(c, fiber.StatusInternalServerError, fmt.Sprintf("Failed to create user: %v", err))
 	}
 
 	// 4. Cleanup Redis