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/resources/views/mail/registration-code.blade.php

@@ -0,0 +1,14 @@
+<x-mail::message>
+# Verification Code
+
+Use the code below to confirm your account:
+
+<x-mail::panel>
+{{ $code }}
+</x-mail::panel>
+
+This code expires in 10 minutes. If you didn't request this, you can safely ignore this email.
+
+Thanks,<br>
+{{ config('app.name') }}
+</x-mail::message>