fix: API 全部改用显式两步走 await session.execute + result.scalars()

之前 (await ...).scalars() 链式在 SQLAlchemy 2.0 async 下报
'coroutine' has no attribute 'scalars' 错误。改为先 await 拿 result
再 .scalars(),这是 SQLAlchemy 2.0 推荐的 async 写法。

backend/app/api/sources.py

@@ -19,7 +19,6 @@ async def list_sources(
     user: User = Depends(get_current_user),  # noqa: ARG001
     session: AsyncSession = Depends(get_session),
 ):
-    rows = (
-        await session.execute(select(Source).order_by(Source.priority.desc(), Source.name))
-    ).scalars()
+    result = await session.execute(select(Source).order_by(Source.priority.desc(), Source.name))
+    rows = result.scalars()
     return [SourceOut.model_validate(s) for s in rows]