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

test_celery_email.py

@@ -273,6 +273,11 @@ def main():
     # 测试4: 发送单个测试邮件（同步）
     logger.info("\n[测试4] 同步发送测试邮件")
     try:
+        # 初始化Django环境
+        os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'diary_family.settings')
+        import django
+        django.setup()
+        
         from django.conf import settings
         from django.core.mail import EmailMessage
         from django.utils import timezone