Checks in an attendee at the specified checkpoint. Looks up the attendee by UUID or email address. Marks the attendee as checked in and returns the updated checked-in count, the attendee ID, and any custom messages.

Fails with 422 if:

• The attendee is not found or does not belong to this event
• The checkpoint has reached capacity
• The attendee is not in confirmed status
• The attendee has an unpaid balance and their attendee type requires payment
• The checkpoint type is 'unique' and the attendee is already checked in
• The attendee's attendee type is not in the checkpoint's allowed list