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/download_attachment.php

@@ -7,6 +7,7 @@
 
 session_start();
 require_once dirname(__DIR__) . '/config/config.php';
+require_once dirname(__DIR__) . '/helpers/Database.php';
 require_once dirname(__DIR__) . '/models/AttachmentModel.php';
 require_once dirname(__DIR__) . '/models/TicketModel.php';
 
@@ -42,24 +43,12 @@ try {
     }
 
     // Verify the associated ticket exists and user has access
-    $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);
-        header('Content-Type: application/json');
-        echo json_encode(['success' => false, 'error' => 'Database connection failed']);
-        exit;
-    }
+    $conn = Database::getConnection();
 
     $ticketModel = new TicketModel($conn);
     $ticket = $ticketModel->getTicketById($attachment['ticket_id']);
 
     if (!$ticket) {
-        $conn->close();
         http_response_code(404);
         header('Content-Type: application/json');
         echo json_encode(['success' => false, 'error' => 'Associated ticket not found']);
@@ -68,7 +57,6 @@ try {
 
     // Check if user has access to this ticket based on visibility settings
     if (!$ticketModel->canUserAccessTicket($ticket, $_SESSION['user'])) {
-        $conn->close();
         http_response_code(403);
         header('Content-Type: application/json');
         echo json_encode(['success' => false, 'error' => 'Access denied to this ticket']);