Add the attribute for a avatar image url for Author.

test/decentralised_book_index/metadata/author_roles_test.exs

@@ -24,4 +24,16 @@ defmodule DecentralisedBookIndex.Metadata.AuthorRolesTest do
       refute author_role.role
     end
   end
+
+  describe "author's avatar image" do
+    test "update avatar image" do
+      avatar_url = "/images/avatar.avif"
+
+      {:ok, author} = Metadata.create_author("Author", "An description")
+
+      assert {:ok, author} = Metadata.assign_author_avatar_image(author, avatar_url)
+
+      assert author.avatar_url == avatar_url
+    end
+  end
 end