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/Models/User.php

@@ -49,6 +49,7 @@ class User extends Authenticatable implements FilamentUser
     protected $fillable = [
         'name',
         'email',
+        'phone',
         'password',
     ];