Introduce testing for backend code

[janine9vn]

Description

We should look into how to test our backend Starlette code to ensure that changes do not invalidate existing validations, features or other components in the application.

Justification

We want to notice when things go awry before we deploy them to production, so we should aim to validate our model code to ensure we don't end up with dodgy data in our database.

Implementation

We'll need to pick a testing framework, Starlette also provides a testing client for automating tests.