add Children to backend

[lkeegan]

We need children logic, data and endpoints in the backend:

Child

• id
• user_id
• name
• photo? <-- note: shouldn't be a public image file!
• date of birth

CRUD endpoints authenticated by user with access of children of this user

[lkeegan]

Photo could be included directly as a uri in the json, but this is slow & inefficient.

Probably makes more sense to have a separate endpoint to ask for the image for a child, which authenticates that the user owns the child before returning the image as a https://fastapi.tiangolo.com/advanced/custom-response/#fileresponse