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>

controllers/TicketController.php

@@ -176,25 +176,32 @@ class TicketController {
             }
 
             // Update ticket with user tracking
-            $result = $this->ticketModel->updateTicket($data, $userId);
+            // Pass expected_updated_at for optimistic locking if provided
+            $expectedUpdatedAt = $data['expected_updated_at'] ?? null;
+            $result = $this->ticketModel->updateTicket($data, $userId, $expectedUpdatedAt);
 
             // Log ticket update to audit log
-            if ($result && isset($GLOBALS['auditLog']) && $userId) {
+            if ($result['success'] && isset($GLOBALS['auditLog']) && $userId) {
                 $GLOBALS['auditLog']->logTicketUpdate($userId, $id, $data);
             }
 
             // Return JSON response
             header('Content-Type: application/json');
-            if ($result) {
+            if ($result['success']) {
                 echo json_encode([
                     'success' => true,
                     'status' => $data['status']
                 ]);
             } else {
-                echo json_encode([
+                $response = [
                     'success' => false,
-                    'error' => 'Failed to update ticket'
-                ]);
+                    'error' => $result['error'] ?? 'Failed to update ticket'
+                ];
+                if (!empty($result['conflict'])) {
+                    $response['conflict'] = true;
+                    $response['current_updated_at'] = $result['current_updated_at'] ?? null;
+                }
+                echo json_encode($response);
             }
         } else {
             // For direct access, redirect to view