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>

cron/create_recurring_tickets.php

@@ -74,7 +74,7 @@ try {
 
                 // Assign to user if specified
                 if ($recurring['assigned_to']) {
-                    $ticketModel->updateTicket($ticketId, ['assigned_to' => $recurring['assigned_to']]);
+                    $ticketModel->assignTicket($ticketId, $recurring['assigned_to'], $recurring['created_by']);
                 }
 
                 // Log to audit