Establish initial database schema

[pbiology]

Description

Establish and describe the database schema to be used in the backend. While this might change in the future, we need to create a first version which is a mature as possible.

Suggested solution

This can be closed when

• [ ] A schema has been decided upon
• [ ] The schema is described and accessible somewhere in this repo

Blocked by

• [x] #5

[pbiology]

Some ideas have been collaborated on here: https://jsoncrack.com/editor?json=c81da5a3ae58a16a831c823f