Cached proxy messages are wrongly "you-responsive" aka. flaky `features/federation/chat.feature:73`

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features/federation/chat.feature:73 is flaky since some time:

Steps to reproduce

1. As UserA on ServerA invite UserB1 from ServerB and UserB2 from ServerB to a chat
2. As UserB1 post a message and delete it
3. As UserB1 (BENNET from the screenshot) check left sidebar and chat for the delete message:
   
4. As UserB2 (NOT BENNET from the screenshot) check left sidebar and chat for the delete message:
   
5. If it looks correct, repeat with UserB2 as author and actor

Actual behaviour

At least for one of the UserB* there should be a wrong left sidebar message. The question is whether the message was federated for the first or second user quicker, or for the actor as a response for their action.

In the chat view it's all correct as we proxy the message request and can therefore correctly do the "you" analysis.
But in the cached PCM table we have only 1 message for all users. I guess we should use the non-you version for now, until we can fix this properly?

Start with features/federation/chat.feature:73 and reproduce the scenario using two invited users from the remote server, including swapping the message author. Compare the left-sidebar cached message with the chat view; done means the sidebar no longer shows an incorrect "you"-responsive message for either user.