Build a small fake invoice service in a new practice folder, running on this computer only. It hands out two made-up
invoices to whoever shows the right made-up key, and refuses everyone else with a short "no access".
Then build a read-only connection that reads its key from a protected settings file, fetches the invoices and prints one line
per invoice. It never sends, changes or deletes anything, and never prints the key. No real account, no internet.
Show me how I check it: two invoice lines with the right key, a clear refusal with a wrong key, a refusal for a request
without any key, and a refusal when I try to send a change with the right key. Stop the service afterwards.
