We first implemented this code in the Hackathon, and forgot to reconsider it later.

We should avoid trusting the user's claims about FIDs. Instead, we should query the identity service.

This diff avoids changing the API, but updates the implementation to ignore the FIDs passed by the client.

Depends on D13435

I haven't done this yet, but I need to confirm that the Farcaster mutual logic still works correctly