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/assign_ticket.php

@@ -5,6 +5,7 @@ RateLimitMiddleware::apply('api');
 
 session_start();
 require_once dirname(__DIR__) . '/config/config.php';
+require_once dirname(__DIR__) . '/helpers/Database.php';
 require_once dirname(__DIR__) . '/models/TicketModel.php';
 require_once dirname(__DIR__) . '/models/AuditLogModel.php';
 require_once dirname(__DIR__) . '/models/UserModel.php';
@@ -40,18 +41,8 @@ if (!$ticketId) {
     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) {
-    echo json_encode(['success' => false, 'error' => 'Database connection failed']);
-    exit;
-}
+// Use centralized database connection
+$conn = Database::getConnection();
 
 $ticketModel = new TicketModel($conn);
 $auditLogModel = new AuditLogModel($conn);
@@ -69,7 +60,6 @@ if ($assignedTo === null || $assignedTo === '') {
     $targetUser = $userModel->getUserById($assignedTo);
     if (!$targetUser) {
         echo json_encode(['success' => false, 'error' => 'Invalid user ID']);
-        $conn->close();
         exit;
     }
 
@@ -80,6 +70,4 @@ if ($assignedTo === null || $assignedTo === '') {
     }
 }
 
-$conn->close();
-
 echo json_encode(['success' => $success]);