Implement Phase 2: Billing & Subscriptions

Add complete billing system with Stripe and PayPal gateway support,
checkout flow with coupon validation, subscription management
(cancel/resume/swap), payment method management, invoice and
transaction history, webhook handlers, dunning/suspension system
with scheduled processing, and 29 new tests (53 total passing).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

website/app/Providers/AppServiceProvider.php

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace App\Providers;
 
 use App\Http\Responses\LoginResponse;
+use App\Services\Billing\BillingServiceFactory;
 use Illuminate\Cache\RateLimiting\Limit;
 use Illuminate\Http\Request;
 use Illuminate\Support\Facades\RateLimiter;
@@ -17,6 +18,7 @@ class AppServiceProvider extends ServiceProvider
     public function register(): void
     {
         $this->app->singleton(LoginResponseContract::class, LoginResponse::class);
+        $this->app->singleton(BillingServiceFactory::class);
     }
 
     public function boot(): void