Offline checkout and sync

What the scanner does when the internet goes out and how to resolve conflicts when the connection returns.

Offline list

The staff application retrieves the list of valid tickets. Without internet, scanning uses this list and saves operations to a local queue.

When the connection is regained, the queue is sent with idempotency keys, so the repeated operation will not be written twice.

If the ticket has been used elsewhere in the meantime, the staff will see a conflict to resolve. The system does not silently select the "last save".