Update AuthorRole delete logic to cascade if no Book.

lib/decentralised_book_index/metadata/author_role.ex

@@ -14,6 +14,10 @@ defmodule DecentralisedBookIndex.Metadata.AuthorRole do
   postgres do
     table "author_role"
     repo DecentralisedBookIndex.Repo
+
+    references do
+      reference :book, index?: true, on_delete: :delete
+    end
   end
 
   actions do