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/Http/Requests/VerifyRegistrationCodeRequest.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Modules\Identity\Http\Requests;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class VerifyRegistrationCodeRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /**
+     * @return array<string, array<int, string>>
+     */
+    public function rules(): array
+    {
+        return [
+            'identifier' => ['required', 'string'],
+            'code' => ['required', 'string', 'size:6'],
+        ];
+    }
+}