Enhancement: Add scanning per user

We should be able to somehow add customers to the database and scan their items in such a way that we can look up a customer's order/purchase history.

The issue names no files, tests, or entry points. Start by reading the C# WPF application's existing inventory and database flows to determine where customer records, item scanning, and purchase history belong. Done means customers can be added, their items can be scanned, and their order or purchase history can be looked up.

Written by the indexing model from the issue text.