ADMIN

Starts an asynchronous batch that removes every voucher carrying this
batch's tag. Returns immediately with the deleting batch; poll it for
progress.

Redeemed vouchers are left in place and counted as skipped. Removing
one would destroy the record that the code was used, and would let the same
code become redeemable again if the drop were re-imported. Set
include_redeemed when the batch was a genuine mistake and the redemptions
are test noise — that removes them too, and cannot be undone.

The batch itself is retained after its vouchers are gone, as the record that
the drop existed.