Fix missing order_number when admin places order for customer

The orders table requires order_number (non-nullable, unique) but the
placeOrder method didn't set it, causing a QueryException.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

website/app/Http/Controllers/Admin/CustomerController.php

@@ -314,6 +314,7 @@ class CustomerController extends Controller
             $order = Order::query()->create([
                 'user_id' => $user->id,
                 'plan_id' => $plan->id,
+                'order_number' => 'ORD-'.strtoupper(Str::random(10)),
                 'total' => $plan->price,
                 'status' => 'completed',
                 'payment_method' => 'admin_created',