Update DBIServer to make the relationship to another server public.

lib/decentralised_book_index/metadata/dbi_server.ex

@@ -103,7 +103,9 @@ defmodule DecentralisedBookIndex.Metadata.DBIServer do
   end
 
   relationships do
-    belongs_to :dbi_server, Metadata.DBIServer
+    belongs_to :dbi_server, Metadata.DBIServer do
+      public? true
+    end
 
     has_many :author, Metadata.Author
     has_many :author_alias_registries, Metadata.AuthorAliasRegistry