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/delete_comment.php

@@ -16,6 +16,7 @@ ob_start();
 
 try {
     require_once dirname(__DIR__) . '/config/config.php';
+    require_once dirname(__DIR__) . '/helpers/Database.php';
     require_once dirname(__DIR__) . '/models/CommentModel.php';
     require_once dirname(__DIR__) . '/models/AuditLogModel.php';
 
@@ -39,17 +40,8 @@ try {
     $userId = $currentUser['user_id'];
     $isAdmin = $currentUser['is_admin'] ?? false;
 
-    // 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) {
-        throw new Exception("Database connection failed");
-    }
+    // Use centralized database connection
+    $conn = Database::getConnection();
 
     // Get data - support both POST body and query params
     $data = json_decode(file_get_contents('php://input'), true);