implement SignupUser two-step confirm flow

Signup now collects only email + displayName, creates an
unconfirmed user with empty password hash, mints an
EmailConfirmationToken, and dispatches a confirmation email.
Password is set during ConfirmUserEmail.

backend/app/User/UseCases/SignupUser/SignupUserRequest.php

@@ -7,6 +7,5 @@ class SignupUserRequest
     public function __construct(
         public ?string $email,
         public ?string $displayName,
-        public ?string $password,
     ) {}
 }