Elektra: Use Case: Register user account

Elektra 0.8.24

Use Case: Register user account

## Summary

- Title: Register user account
- Scope: Authentication
- Level: User Goal
- Actors: Anonymous user
- Brief: Any not logged in user can register a new user account.

## Scenarios

- Precondition:
- Main success scenario: The user successfully created a new account. The user is informed about the success.
- Alternative scenario: The inputs could not be validated (do not match the required constraints). The user is informed about their mistake.
- Error scenario: Technical problems prevent the registration from succeeding. The user is informed about the issue.
- Postcondition: The user account is created successfully in the database and the provided credentials can now be used to authenticate against the service.
- Non-functional Constraints:
  - Security mechanism
  - Is precondition to use other functions