Add phone-verified user registration

Two-step flow: request a one-time code by email/phone (RegistrationVerification,
mailed via RegistrationCodeMail, rate-limited by the new api-otp limiter keyed
to the identifier), then verify the code and register with RegistrationController.
User gains a phone column/fillable.

app-modules/identity/src/Mail/RegistrationCodeMail.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Modules\Identity\Mail;
+
+use Illuminate\Bus\Queueable;
+use Illuminate\Mail\Mailable;
+use Illuminate\Queue\SerializesModels;
+
+class RegistrationCodeMail extends Mailable
+{
+    use Queueable, SerializesModels;
+
+    public function __construct(public readonly string $code) {}
+
+    public function build(): self
+    {
+        return $this
+            ->subject(config('app.name').' - Verification Code')
+            ->view('identity::mail.registration-code');
+    }
+}