refactor(system): 重构消息中心功能

- 移除 WebSocket相关代码
- 新增 SSE (Server-Sent Events) 实现消息推送
- 优化消息中心未读数量展示和更新逻辑- 调整消息中心相关 API 和前端展示

backend/dvadmin/system/signals.py

@@ -1,4 +1,10 @@
-from django.dispatch import Signal
+import time
+
+from django.db.models.signals import post_save, post_delete
+from django.dispatch import Signal, receiver
+from django.core.cache import cache
+from dvadmin.system.models import MessageCenterTargetUser
+
 # 初始化信号
 pre_init_complete = Signal()
 detail_init_complete = Signal()
@@ -10,3 +16,12 @@ post_tenants_init_complete = Signal()
 post_tenants_all_init_complete = Signal()
 # 租户创建完成信号
 tenants_create_complete = Signal()
+
+# 全局变量用于标记最后修改时间
+last_db_change_time = time.time()
+
+
+@receiver(post_save, sender=MessageCenterTargetUser)
+@receiver(post_delete, sender=MessageCenterTargetUser)
+def update_last_change_time(sender, **kwargs):
+    cache.set('last_db_change_time', time.time(), timeout=None)  # 设置永不超时的键值对