perf: 优化验证码计算结果为非负数 (#30)

Author: insistence

ruoyi-fastapi-backend/module_admin/service/captcha_service.py

@@ -34,6 +34,10 @@ async def create_captcha_image_service(cls) -> list[str, int]:
             result = num1 + num2
         elif operational_character == '-':
             result = num1 - num2
+            # 非负数结果的验证码更友好一些
+            if result < 0:
+                num1, num2 = num2, num1
+                result = num1 - num2
         else:
             result = num1 * num2
         # 绘制文本