db: add constraints to role tables

app/models/expert.rb

@@ -5,7 +5,7 @@
 #  id         :bigint           not null, primary key
 #  created_at :datetime         not null
 #  updated_at :datetime         not null
-#  user_id    :bigint
+#  user_id    :bigint           not null
 #
 class Expert < ApplicationRecord
   belongs_to :user