Fix `APIUser` and `RealmUser` silently desyncing on spectator-tracked scores

[bdach]

Found while testing https://github.com/ppy/osu/pull/28119.

Please read diatribe attached to test case for explanation.

"Fortunately" this does not affect replays being recorded right now because by sheer luck the only thing encoded to replay off of any of these models right now is the username and it uses User which is the "correct" one.