[Check-in] Allow for Scanning Terminal API

"it'd be great if attend exposed an API for checking people in. we thought of using PCB event badges that advertise over BLE with a scanner to very quickly check people in during fallout but found out it isn't possible because attend doesn't have an api for that D:

minimally methinks this needs an endpoint that takes an attendee UUID and a context, plus token auth for the scanner"

The issue names no files or tests; start by locating the existing check-in flow and API entry points, then review how attendee UUIDs and authentication are handled. Done means a scanner can use a token-authenticated endpoint with an attendee UUID and context to check someone in, with the behavior covered by tests.

Written by the indexing model from the issue text.