refactor: address review comments for join view

lib/membership/join_request.ex

@@ -37,6 +37,7 @@ defmodule Mv.Membership.JoinRequest do
       accept [:email, :first_name, :last_name, :form_data, :schema_version]
 
       change Mv.Membership.JoinRequest.Changes.SetConfirmationToken
+      change Mv.Membership.JoinRequest.Changes.FilterFormDataByAllowlist
     end
 
     read :get_by_confirmation_token_hash do
@@ -77,6 +78,8 @@ defmodule Mv.Membership.JoinRequest do
   end
 
   validations do
+    # Format/formatting of email is not validated here; invalid addresses may fail at send time
+    # or can be enforced via an Ash change if needed.
     validate present(:email), on: [:create]
   end