[Feature]: Complete sign up logic

[Slartibartfass2]

To which use case does your feature request relate?

#2

Is your feature request related to a problem? Please describe

Currently, the sign-up endpoint doesn't work properly and needs to be fixed. Until this is fixed, we cannot implement the correct sign-up behavior.

Describe the solution you'd like

Wait for SE-UUlm/snowballr-backend#267 and then complete the sign-up logic.

Describe alternatives you've considered

No response

Anything else?

No response