It is observed for each redemption group and it could vary for groups
belonging to a single voucher.

Take care not to spend them, though.

Instead of immediately extracting passes, get them and later mark up the store
with the outcome of the attempt to use them.

This makes the database responsible for remembering how many tokens we expect
to get for a voucher, rather than the in-memory Python state or the code.

As a result, we can resume redemption knowing the correct number of tokens
after a process restart and without relying on the PaymentController's default
token count to have remained unchanged.

Allow voucher additions that are only distinct on the counter value.

A new voucher has not been redeemed any times.

It is stored on redemption and regurgitated by the Python APIs and dumped in
the JSON accessible via the HTTP API.