Point in-app wallet top-up at the billing API (key the app already holds)

qr-plus authenticates to the monolith with a billing service key, not an
identity key, so the modal's top-up now goes through BillingClient::topup ->
POST /api/billing/topup. Drops the unused IdentityClient::walletTopup.

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

app/Http/Controllers/WalletTopupController.php

@@ -2,7 +2,7 @@
 
 namespace App\Http\Controllers;
 
-use App\Services\Identity\IdentityClient;
+use App\Services\Billing\BillingClient;
 use Illuminate\Http\JsonResponse;
 use Illuminate\Http\RedirectResponse;
 use Illuminate\Http\Request;
@@ -15,7 +15,7 @@ use Illuminate\Http\Request;
  */
 class WalletTopupController extends Controller
 {
-    public function store(Request $request, IdentityClient $identity): JsonResponse|RedirectResponse
+    public function store(Request $request, BillingClient $billing): JsonResponse|RedirectResponse
     {
         $validated = $request->validate([
             'amount' => ['required', 'numeric', 'min:5', 'max:5000'],
@@ -25,7 +25,7 @@ class WalletTopupController extends Controller
         $returnUrl = $validated['return_url'] ?? (string) (url()->previous() ?: url('/'));
 
         try {
-            $checkoutUrl = $identity->walletTopup(
+            $checkoutUrl = $billing->topup(
                 (string) $request->user()->public_id,
                 (float) $validated['amount'],
                 $returnUrl,