[FEAT] Add testing for API

[Nipa-Dev]

Add testing to lemonapi

Currently the API is missing testing and lack of testing can make it hard to ensure that the app works as it should. Including tests with FastAPI TestClient and testing at least the primary endpoints and their functionality makes it easier to notice if something doesn't work.

Minimal list of tasks

Writing tests for endpoints to make sure:

• database connection works as it should
• user authentication works
• can access protected/public endpoints without problem
• endpoints return the correct data you would expect

Other

Aim for full coverage of endpoints to make sure everything works

[realstealthninja]

on it