fix: auto-fix code issues (cron)

- 修复重复导入/字段
- 修复异常处理
- 修复PEP8格式问题
- 添加类型注解

backend/db_manager.py

@@ -123,7 +123,7 @@ class DatabaseManager:
         conn.row_factory = sqlite3.Row
         return conn
 
-    def init_db(self):
+    def init_db(self) -> None:
         """初始化数据库表"""
         with open(os.path.join(os.path.dirname(__file__), "schema.sql")) as f:
             schema = f.read()
@@ -299,7 +299,7 @@ class DatabaseManager:
         conn.close()
         return self.get_entity(entity_id)
 
-    def delete_entity(self, entity_id: str):
+    def delete_entity(self, entity_id: str) -> None:
         """删除实体及其关联数据"""
         conn = self.get_conn()
         conn.execute("DELETE FROM entity_mentions WHERE entity_id = ?", (entity_id,))