fix: 修复邮件后端导入并添加Django环境初始化

修复了core/tasks.py中错误的EmailBackend导入方式，改为从正确路径导入。同时在测试文件中添加了Django环境初始化代码，确保测试能够正确运行。

core/tasks.py

@@ -1,6 +1,7 @@
 from celery import shared_task
 from loguru import logger
-from django.core.mail import EmailMessage, EmailMultiAlternatives, EmailBackend
+from django.core.mail import EmailMessage, EmailMultiAlternatives
+from django.core.mail.backends.smtp import EmailBackend
 from django.utils import timezone
 from datetime import timedelta
 import traceback