profileUpdatedAt ?? createdAt treated freshly registered users as having
"newer" profiles than users with real edit history, because createdAt is
always NOW at registration time. This broke federation profile sync:
the client correctly pushed home → remote, but the remote server's LWW
guard rejected the write (stored createdAt > incoming profileUpdatedAt).
A null profileUpdatedAt means "never edited" — that's timestamp 0, not
the user's creation time.