Bulk-create promotional codes

Creates many promotions in a single request, all sharing the configuration in promotion. Supply the codes explicitly via codes, OR ask the server to generate them via generate (exactly one of the two is required). Up to 1000 codes may be created per request.

The response reports partial success: every requested code appears either in created or in failed with a reason (e.g. already taken, duplicate in request). A 201 is returned when all codes were created, 207 when some failed.