feat(backend): feed items carry the message id even when the message row is gone

backend/src/utils/read/models.py

@@ -226,6 +226,7 @@ class FeedItem(BaseModel):
     since: datetime
     message_count: int
     chat: FeedChat
+    message_id: int
     message: MessageView | None
     sender_id: int | None
     sender_name: str | None