Make all HTTP endpoint indempotent

[Indy2222]

Due to e.g. network issues, some requests might need to be retried. To avoid potential retry errors, make all endpoint idempotent.

For example a POST request to create a game should not result in an error if it leads to the create of a game with the same configuration and owner.

[github-actions]

There is no activity on the issue for some time.