General attendees who haven't registered and haven't paid can still be checked-in

[danielamassiceti]

If a general attendee hasn't completed the registration form (i.e. there is no entry for them in the 'registration' table) and they have not paid their attendance fee (i.e. confirmed = False), they can still be checked-in (i.e. added to the 'attendance' table).

This is okay when all attendees have completed their registration forms, but is not okay if this is not the case.

Ideal solution:

• Rather than extracting the 'confirmed' field from a registration object, the GET in the AttendanceAPI should query the 'invoice' table to check if the payment amount of the offer matches the required payment amount (or at least is not null or 0)
• This should be done with as low a query load as possible, as check-in functionality is often used by many clients in parallel in a short space of time