id should be an integer

[rpo19]

https://github.com/UNIMIBInside/IKBP/blob/a671012b1963c3bcb2d7d0e77480d20acbbdf5d6/documents/src/models/annotation.js#L13

@marcoripa96 id should be an integer, we should reason about why it is a string and if it is enough to set it as here.

And maybe also think at how to fix the 6 documents already in the db.

[rpo19]

ho creato 2 repo; per ora finchè non finiamo l'annotazione lasciamo il workaround (che non cambia la struttura db ma fa il cast alla chiamata API)