fix: add error handler to JWT middleware for better API responses

- Improve error response format for authentication failures
- Return consistent JSON error messages
- Enhance API client experience

internal/router/router.go

@@ -83,6 +83,9 @@ func createJWTMiddleware(cfg *Config) echo.MiddlewareFunc {
 				Role:     claims["user_role"].(string),
 			})
 		},
+		ErrorHandler: func(c echo.Context, err error) error {
+			return c.JSON(http.StatusUnauthorized, map[string]string{"error": err.Error()})
+		},
 	})
 }