feat: prevent join requests with equal mail

test/membership/join_request_approval_policy_test.exs

@@ -49,6 +49,7 @@ defmodule Mv.Membership.JoinRequestApprovalPolicyTest do
       assert approved.status == :approved
       assert approved.approved_at != nil
       assert approved.reviewed_by_user_id == user.id
+      assert approved.reviewed_by_display == to_string(user.email)
     end
 
     test "admin can approve a submitted join request", %{request: request} do
@@ -89,6 +90,7 @@ defmodule Mv.Membership.JoinRequestApprovalPolicyTest do
       assert rejected.status == :rejected
       assert rejected.rejected_at != nil
       assert rejected.reviewed_by_user_id == user.id
+      assert rejected.reviewed_by_display == to_string(user.email)
     end
 
     test "admin can reject a submitted join request", %{request: request} do