Implement login functionality in API

[dillu24]

Since we are going to migrate the installer to the UI, it is of paramount importance that the configurations are not exposed to the public. As a result, we need to start implementing a basic login system.

At this point we can go with the following flow:

1. Put temporary username and password inside the .env file for the UI. The user will be asked to change this prior to deployment inside the documentation.
2. On startup, the API loads the username and password inside the database.
3. On UI startup, the user is asked to enter the username and password.
4. The API gives a jwt token to the user, and authentication proceeds using the JWT system.

See the installer backend implementation for more details.