It should not be possible to spectate users that are offline or appear offline

[bdach]

See discussion starting from https://github.com/ppy/osu-server-spectator/pull/259#issuecomment-2597630099.

There's a decent chance doing this will require merging metadata & spectator hubs in some capacity.

[peppy]

I think a good starting point would be handling this client side (while at the spectator screen, send a StopWatching when the user goes offline, and re-watch when they come back online). This would handle all cases where the client is authoritative, ie. the user isn't doing funny-business.