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/Services/Identity/IdentityClient.php

@@ -43,24 +43,6 @@ class IdentityClient
         return (array) $response->json('data', []);
     }
 
-    /**
-     * Start a central-wallet Paystack top-up for an account and return the
-     * Paystack checkout URL. Backs the in-app "Add funds" modal so users top up
-     * without leaving the app for the central wallet page.
-     */
-    public function walletTopup(string $publicId, float $amount, string $returnUrl): string
-    {
-        $response = $this->request()->post($this->url('/identity/wallet-topup-account'), [
-            'user' => $publicId,
-            'amount' => $amount,
-            'return_url' => $returnUrl,
-        ]);
-
-        $response->throw();
-
-        return (string) $response->json('data.checkout_url', '');
-    }
-
     private function request()
     {
         return Http::withToken((string) config('identity.api_key'))