Add endpoint for retrieving demonlist player linked to logged-in account

[stadust]

Add something like GET /auth/me/claimed or similar. We cannot return this as an additional field in /auth/me as claims are a pointercrate-demonlist concept, and /auth/me lives in pointercrate-user (and we neither want nor can add a dependency on pointercrate-demonlist to pointercrate-user). The implementation of the endpoint itself should be almost trivial, e.g. should just be a return PlayerClaim::by_user(auth.user.id)?.

This endpoint also needs to be added to the documentation.

[stadust]

Duplicate of #191, and also resolved by #197