add migration and adapt fixture and factories

spec/factories/article.rb

@@ -7,6 +7,7 @@ FactoryBot.define do
     tax { [6, 21].sample }
     deposit { rand(10) < 8 ? 0 : [0.0, 0.80, 1.20, 12.00].sample }
     unit_quantity { rand(5) < 3 ? 1 : rand(1..20) }
+    price_per { rand(0.1..26.0).round(2) }
 
     factory :article do
       sequence(:name) { |n| Faker::Lorem.words(number: rand(2..4)).join(' ') + " ##{n}" }