Bill transfer storage monthly with a 15-day grace period before deletion.

Files stay available while wallet renewals succeed each month. Failed renewals
keep files for TRANSFER_GRACE_PERIOD_DAYS (default 15) before automatic cleanup.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/Feature/TransferApiTest.php

@@ -8,16 +8,19 @@ use Illuminate\Http\UploadedFile;
 use Illuminate\Support\Facades\Http;
 use Illuminate\Support\Facades\Storage;
 use Illuminate\Support\Str;
+use Tests\Concerns\FakesTransferBilling;
 use Tests\TestCase;
 
 class TransferApiTest extends TestCase
 {
+    use FakesTransferBilling;
     use RefreshDatabase;
 
     protected function setUp(): void
     {
         parent::setUp();
         Storage::fake('qr');
+        $this->fakeTransferBillingApi();
         config([
             'transfer.service_api_keys' => ['webmail' => 'test-webmail-key'],
             'identity.api_url' => 'https://ladill.com/api',