refactor(billing): clean architecture with payment gateway abstraction

- Add PaymentGatewayInterface contract for all payment providers
- Add PayPalGateway (reads credentials from config, not hardcoded)
- Add ManualGateway for admin-triggered payments
- Add StripeGateway stub for future implementation
- Add GatewayFactory for gateway instantiation by name
- Add BillingRepository: parameterized-SQL data layer
- Add BillingService: pricing, invoice creation, payment processing
- Add gsp_billing_transactions table (DB version 2) for audit trail
- Add new columns to gsp_billing_invoices (home_id, rate_type, players, period_start/end, subtotal, total_due, payment_status)
- Add gsp_billing_service_remote_servers mapping table
- Move PayPal credentials from api files into config.inc.php
- Fix double session_start() bug in capture_order.php
- Replace raw SQL with prepared statements throughout
- Refactor admin_invoices.php to use billing_invoices + BillingRepository
- Refactor admin_payments.php to read from gsp_billing_transactions
- Update admin.php with links to Transaction Log and Manage Invoices

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: iaretechnician <2749183+iaretechnician@users.noreply.github.com>

modules/billing/classes/ManualGateway.php

@@ -0,0 +1,36 @@
+<?php
+require_once __DIR__ . '/../classes/PaymentGatewayInterface.php';
+
+/**
+ * Manual / offline payment gateway.
+ * Used when an admin marks a payment as paid directly.
+ */
+class ManualGateway implements PaymentGatewayInterface
+{
+    public function getName(): string { return 'manual'; }
+
+    public function createPayment(array $params): array
+    {
+        return ['success' => true, 'provider_order_id' => 'MANUAL-' . uniqid(), 'raw_response' => []];
+    }
+
+    public function handleCallback(array $params): array
+    {
+        $txid = $params['admin_txid'] ?? ('MANUAL-' . uniqid());
+        return [
+            'success'        => true,
+            'transaction_id' => $txid,
+            'amount'         => (float)($params['amount'] ?? 0),
+            'currency'       => $params['currency'] ?? 'USD',
+            'status'         => 'completed',
+            'raw_response'   => $params,
+        ];
+    }
+
+    public function verifyPayment(array $payload): bool { return true; }
+
+    public function getTransactionId(array $captureResult): ?string
+    {
+        return $captureResult['transaction_id'] ?? null;
+    }
+}