Posso usare l'API di Discourse per autenticare gli utenti in un'altra app?

Are you referring to the API key? It seems possible to create a “granular” API key, that has access to specific API end points only. It is still not clear to me which endpoints would be required, if I use that approach. Do you know?

Yes, a minimal web service with auth-proxy might be a good solution; I’ll have to experiment a bit to find out.