Add a "liveness probe" for score submissions such that outages don't cause users to be unable to play

As other efforts to attempt increasing resiliency turn out to be either a lot of work or stalled due to circumstances outside our control, a short-term solution to enable the game being playable during server outages would be to implement a sort of "liveness probe", being an endpoint independent of other infra that should be as simple and as resilient to falling over as possible, which would be used by the client to determine whether things are alive and whether to bother submitting scores or not.

Have not thought of any of the details like:

• where this should live
• how often to query this probe
• whether it should affect just score submission or e.g. instantly knock the entirety of API into Failing state (you could call it dubious but there's very little in the game right now that can work without web. if you want that, then read https://github.com/ppy/osu-server-spectator/issues/367 and https://github.com/ppy/osu-server-spectator/issues/368#issuecomment-3536556034).

cc @peppy