Assign postpone_by to system when auto-cleaning

app/models/card/entropic.rb

@@ -22,7 +22,7 @@ module Card::Entropic
   class_methods do
     def auto_postpone_all_due
       due_to_be_postponed.find_each do |card|
-        card.postpone
+        card.postpone(user: User.system)
       end
     end
   end

test/models/card/entropic_test.rb

@@ -33,6 +33,7 @@ class Card::EntropicTest < ActiveSupport::TestCase
     end
 
     assert cards(:logo).reload.postponed?
+    assert_equal User.system, cards(:logo).postponed_by
     assert_not cards(:shipping).reload.postponed?
   end