Limit voucher codes to one use per customer

Some providers promote a single voucher code on printed materials (e.g. leaflets), intending it to be used once per customer. At the moment the ‘Number of Uses’ setting limits the total number of redemptions across all customers, not uses per person - so customers can redeem the same shared code multiple times for multiple bookings.

The existing workaround (auto-generating a batch of unique single-use codes) doesn’t work in this scenario, because a code printed on leaflets has to be one shared code that everybody sees.

The request: add a ‘maximum uses per customer’ setting on voucher codes (e.g. 1 per customer), alongside the existing total uses limit. This would let providers safely run a single public code on printed materials without it being reused.