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/PaymentGatewayInterface.php

@@ -0,0 +1,40 @@
+<?php
+/**
+ * Payment Gateway Interface
+ * All payment providers must implement this contract.
+ */
+interface PaymentGatewayInterface
+{
+    /**
+     * Create a payment/order on the provider side.
+     * @param array $params { amount, currency, invoice_id, description, return_url, cancel_url, items? }
+     * @return array { success: bool, provider_order_id: string, redirect_url?: string, error?: string }
+     */
+    public function createPayment(array $params): array;
+
+    /**
+     * Handle a provider callback/capture (webhook or return).
+     * @param array $params Provider-specific parameters (e.g. { order_id } for PayPal)
+     * @return array { success: bool, transaction_id: string, amount: float, status: string, raw_response: array, error?: string }
+     */
+    public function handleCallback(array $params): array;
+
+    /**
+     * Verify that a payment/webhook is authentic.
+     * @param array $payload  Raw request body / headers
+     * @return bool
+     */
+    public function verifyPayment(array $payload): bool;
+
+    /**
+     * Get the provider's external transaction ID from a capture result.
+     * @param array $captureResult Result from handleCallback()
+     * @return string|null
+     */
+    public function getTransactionId(array $captureResult): ?string;
+
+    /**
+     * Return a short machine name for this gateway (e.g. 'paypal', 'stripe', 'manual').
+     */
+    public function getName(): string;
+}