Add unique constraint to Translation

To make sure only one translation per article is possible.

priv/repo/migrations/20221230170041_add_lang_article_id_unique_index_to_translations.exs

@@ -0,0 +1,7 @@
+defmodule Outlook.Repo.Migrations.AddLangArticleIdUniqueIndexToTranslations do
+  use Ecto.Migration
+
+  def change do
+    create unique_index(:translations, [:article_id, :lang], name: :article_id_lang_unique_index)
+  end
+end