BuyerEmailDomain: add unique constraint to domain_name to match migration state
BuyerEmailDomain: add unique constraint to domain_name to match migration state

presumably this got missed off the model definition, but was included in
the migrations and as such we are currently operating as though it were set

alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api