fix(auth): reject signup with an existing customer email (account-claim guard)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Actions/Fortify/CreateNewUser.php

@@ -2,6 +2,7 @@
 
 namespace App\Actions\Fortify;
 
+use App\Models\Customer;
 use App\Models\User;
 use Illuminate\Support\Facades\Hash;
 use Illuminate\Support\Facades\Validator;
@@ -30,6 +31,10 @@ class CreateNewUser implements CreatesNewUsers
                 'email',
                 'max:255',
                 Rule::unique(User::class),
+                // Never let public signup claim an existing (possibly not-yet-
+                // provisioned) customer's email — ensureUser() would later link
+                // that account and hand over the customer's portal.
+                Rule::unique(Customer::class, 'email'),
             ],
             'password' => $this->passwordRules(),
         ])->validate();

tests/Feature/Auth/RegisterTest.php

@@ -24,6 +24,17 @@ it('registers a new account and signs in', function () {
     $this->assertAuthenticatedAs($user);
 });
 
+it('refuses to register with an existing customer email (account-claim guard)', function () {
+    \App\Models\Customer::factory()->create(['email' => 'claim@signup.test']);
+
+    $this->post(route('register.store'), [
+        'name' => 'Claimer', 'email' => 'claim@signup.test',
+        'password' => 'password1234', 'password_confirmation' => 'password1234',
+    ])->assertSessionHasErrors('email');
+
+    expect(User::query()->where('email', 'claim@signup.test')->exists())->toBeFalse();
+});
+
 it('rejects a mismatched password confirmation', function () {
     $this->post(route('register.store'), [
         'name' => 'X',