虚拟退款获取 access_token 时显式导入 cache，避免未导入报错。

Co-authored-by: Cursor <cursoragent@cursor.com>

yonghu/views.py

@@ -9507,6 +9507,7 @@ class KefuPlatformRefundView(APIView):
 
     def _get_access_token(self):
         """获取 access_token，带缓存"""
+        from django.core.cache import cache
         cache_key = "virtual_payment_access_token"
         access_token = cache.get(cache_key)
         if not access_token: