Add a simple authentication

[mateuszkornecki]

Is your feature request related to a problem? Please describe.
Since we have a two different UserRoles we need somehow check i=(for example with PIN).

Describe the solution you'd like
if someones who want to edit his exercise config is a therapist (have sufficient right/knowledge to do it) and ask for the a 4-numbers PIN-password.

Later on we will create the way to login in you are a therapist, but for the MVP just a simple verification should be enough.

Additional context
https://nextjs.org/docs/pages/building-your-application/routing/authenticating