加入部分消息通知入口，同步前端管理

backend/shared/models/system_config.py

@@ -0,0 +1,17 @@
+"""SystemConfig ORM model - 系统配置键值表。"""
+
+from sqlalchemy import Column, DateTime, String
+from sqlalchemy.sql import func
+
+from .base import Base
+
+
+class SystemConfig(Base):
+    __tablename__ = "system_config"
+
+    key = Column(String(64), primary_key=True)
+    value = Column(String(500), nullable=False, default="")
+    updated_at = Column(DateTime, server_default=func.now(), onupdate=func.now())
+
+    def __repr__(self):
+        return f"<SystemConfig(key='{self.key}', value='{self.value[:30]}')>"