Change API to enable auto OpenAPI documentation

[alexgarel]

FastAPI is able to generate OpenAPI documentation from the code, and serve it.

But for this to work:

• POST api methods should describe their body parameters
• they should also use models to return values

We should refactor the code to use these features.

[Anway-Agte]

Can I work on this issue ?

[alexgarel]

@Anway-Agte you are welcome :-)