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/GatewayFactory.php

@@ -0,0 +1,30 @@
+<?php
+require_once __DIR__ . '/../classes/PaymentGatewayInterface.php';
+require_once __DIR__ . '/../classes/PayPalGateway.php';
+require_once __DIR__ . '/../classes/ManualGateway.php';
+require_once __DIR__ . '/../classes/StripeGateway.php';
+
+/**
+ * Factory for instantiating payment gateways by name.
+ */
+class GatewayFactory
+{
+    /**
+     * @param string $name  Gateway name: 'paypal', 'stripe', 'manual'
+     * @return PaymentGatewayInterface
+     * @throws InvalidArgumentException
+     */
+    public static function make(string $name): PaymentGatewayInterface
+    {
+        switch (strtolower($name)) {
+            case 'paypal':
+                return PayPalGateway::fromConfig();
+            case 'manual':
+                return new ManualGateway();
+            case 'stripe':
+                return new StripeGateway();
+            default:
+                throw new InvalidArgumentException("Unknown payment gateway: {$name}");
+        }
+    }
+}