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/includes/config.inc.php

@@ -29,4 +29,9 @@ $SITE_BACKGROUND = trim((string)$SITE_BACKGROUND);
 
 // Data directory for persisted payment webhook JSON files (relative to repo root)
 $SITE_DATA_DIR = realpath(__DIR__ . '/..') . DIRECTORY_SEPARATOR . 'data';
+
+// PayPal configuration — set credentials here, never in API files
+$paypal_sandbox       = true;   // Set to false for live payments
+$paypal_client_id     = '';     // Your PayPal Client ID
+$paypal_client_secret = '';     // Your PayPal Client Secret
 ?>