Add created_at and updated_at fields to Entity dataclass

backend/db_manager.py

@@ -34,6 +34,8 @@ class Entity:
     aliases: List[str] = None
     embedding: str = ""  # Phase 3: 实体嵌入向量
     attributes: Dict = None  # Phase 5: 实体属性
+    created_at: str = ""
+    updated_at: str = ""
     
     def __post_init__(self):
         if self.aliases is None: