feat(backend): backfill error code on legacy error responses

backend/internal/middleware/error_code_enricher.go

@@ -0,0 +1,49 @@
+package middleware
+
+import (
+	"baron-sso-backend/internal/response"
+	"encoding/json"
+	"strings"
+
+	"github.com/gofiber/fiber/v2"
+)
+
+// ErrorCodeEnricher injects machine-readable `code` into legacy error responses.
+// This keeps backward compatibility (`error` stays) while migrating handlers
+// incrementally to explicit code-based responses.
+func ErrorCodeEnricher() fiber.Handler {
+	return func(c *fiber.Ctx) error {
+		err := c.Next()
+
+		body := c.Response().Body()
+		if len(body) == 0 {
+			return err
+		}
+
+		contentType := string(c.Response().Header.ContentType())
+		if contentType != "" && !strings.Contains(contentType, fiber.MIMEApplicationJSON) {
+			return err
+		}
+
+		var payload map[string]any
+		if unmarshalErr := json.Unmarshal(body, &payload); unmarshalErr != nil {
+			return err
+		}
+
+		if _, hasError := payload["error"]; !hasError {
+			return err
+		}
+		if _, hasCode := payload["code"]; hasCode {
+			return err
+		}
+
+		payload["code"] = response.StatusCode(c.Response().StatusCode())
+		updated, marshalErr := json.Marshal(payload)
+		if marshalErr != nil {
+			return err
+		}
+
+		c.Response().SetBodyRaw(updated)
+		return err
+	}
+}