feat(database): bound async configuration writes

app/application/database.py

@@ -18,6 +18,14 @@ DatabaseProbe = Callable[[], Optional[str]]
 T = TypeVar("T")
 
 
+class DatabaseWorkerClosedError(RuntimeError):
+    """数据库执行器尚未启动或已经停止。"""
+
+
+class DatabaseWorkerOverloadedError(RuntimeError):
+    """数据库执行器的运行与排队容量已经用尽。"""
+
+
 class AsyncDatabaseExecutor(Protocol):
     """让异步业务调用同步短事务而不阻塞事件循环。"""