Authenticate API requests

Currently, anyone on the network can access the APIs. This is acceptable for local development (because dev machines are generally behind NAT/firweall), but we'll eventually need to:
- Require that API calls include credentials (eg username+password)
- Update the README to show admins how to register a username+password.

Start by locating the API entry points and the README in the Prevem repository. Review how API requests are currently handled and how users or admins are configured. Done means API calls require credentials and the README explains how an administrator registers a username and password.

Written by the indexing model from the issue text.