Add performance, security, and reliability improvements

- Consolidate all 20 API files to use centralized Database helper
- Add optimistic locking to ticket updates to prevent concurrent conflicts
- Add caching to StatsModel (60s TTL) for dashboard performance
- Add health check endpoint (api/health.php) for monitoring
- Improve rate limit cleanup with cron script and efficient DirectoryIterator
- Enable rate limit response headers (X-RateLimit-*)
- Add audit logging for workflow transitions
- Log Discord webhook failures instead of silencing
- Fix visibility check on export_tickets.php
- Add database migration system with performance indexes
- Fix cron recurring tickets to use assignTicket method

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

api/audit_log.php

@@ -6,6 +6,7 @@
  */
 
 require_once dirname(__DIR__) . '/config/config.php';
+require_once dirname(__DIR__) . '/helpers/Database.php';
 require_once dirname(__DIR__) . '/models/AuditLogModel.php';
 
 session_start();
@@ -26,19 +27,8 @@ if (!$isAdmin) {
     exit;
 }
 
-// Create database connection
-$conn = new mysqli(
-    $GLOBALS['config']['DB_HOST'],
-    $GLOBALS['config']['DB_USER'],
-    $GLOBALS['config']['DB_PASS'],
-    $GLOBALS['config']['DB_NAME']
-);
-
-if ($conn->connect_error) {
-    http_response_code(500);
-    echo json_encode(['success' => false, 'error' => 'Database connection failed']);
-    exit;
-}
+// Use centralized database connection
+$conn = Database::getConnection();
 
 $auditLogModel = new AuditLogModel($conn);
 
@@ -90,7 +80,6 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
         }
 
         fclose($output);
-        $conn->close();
         exit;
     }