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/subscriptions.py

@@ -38,13 +38,12 @@ async def list_mine(
     user: User = Depends(get_current_user),
     session: AsyncSession = Depends(get_session),
 ):
-    rows = (
-        await session.execute(
-            select(Subscription)
-            .where(Subscription.user_id == user.id)
-            .order_by(Subscription.created_at.desc())
-        )
-    ).scalars()
+    result = await session.execute(
+        select(Subscription)
+        .where(Subscription.user_id == user.id)
+        .order_by(Subscription.created_at.desc())
+    )
+    rows = result.scalars()
     return [SubscriptionOut.model_validate(s) for s in rows]