Add a new source=post_replies to /stream/users with a new param ...&post_id={string}&author_id={string} (mimicking current stream of posts of PostReplies).

Can be implemented with currently existing indexes. We just need to retrieve the Sorted set of replies of that post_key , subset the user_id: part of the found members and return unique user_ids to the UserStream model to craft a stream of user profiles for the existing.

Alternatively, we could also simply return the user_ids and assume the client has already cached the profiles.