Use best-effort guessing the url of the outbox for known users

priv/repo/migrations/20241027040000_users_add_outboxes.exs

@@ -5,6 +5,8 @@ def up do
     alter table(:users) do
       add_if_not_exists(:outbox, :text)
     end
+
+    execute("update users set outbox = replace(inbox, 'inbox', 'outbox')")
   end
 
   def down do