Implement comprehensive improvement plan (Phases 1-6)

Security (Phase 1-2):
- Add SecurityHeadersMiddleware with CSP, X-Frame-Options, etc.
- Add RateLimitMiddleware for API rate limiting
- Add security event logging to AuditLogModel
- Add ResponseHelper for standardized API responses
- Update config.php with security constants

Database (Phase 3):
- Add migration 014 for additional indexes
- Add migration 015 for ticket dependencies
- Add migration 016 for ticket attachments
- Add migration 017 for recurring tickets
- Add migration 018 for custom fields

Features (Phase 4-5):
- Add ticket dependencies with DependencyModel and API
- Add duplicate detection with check_duplicates API
- Add file attachments with AttachmentModel and upload/download APIs
- Add @mentions with autocomplete and highlighting
- Add quick actions on dashboard rows

Collaboration (Phase 5):
- Add mention extraction in CommentModel
- Add mention autocomplete dropdown in ticket.js
- Add mention highlighting CSS styles

Admin & Export (Phase 6):
- Add StatsModel for dashboard widgets
- Add dashboard stats cards (open, critical, unassigned, etc.)
- Add CSV/JSON export via export_tickets API
- Add rich text editor toolbar in markdown.js
- Add RecurringTicketModel with cron job
- Add CustomFieldModel for per-category fields
- Add admin views: RecurringTickets, CustomFields, Workflow,
  Templates, AuditLog, UserActivity
- Add admin APIs: manage_workflows, manage_templates,
  manage_recurring, custom_fields, get_users
- Add admin routes in index.php

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

api/add_comment.php

@@ -3,6 +3,10 @@
 ini_set('display_errors', 0);
 error_reporting(E_ALL);
 
+// Apply rate limiting
+require_once dirname(__DIR__) . '/middleware/RateLimitMiddleware.php';
+RateLimitMiddleware::apply('api');
+
 // Start output buffering to capture any errors
 ob_start();
 
@@ -70,12 +74,39 @@ try {
     $commentModel = new CommentModel($conn);
     $auditLog = new AuditLogModel($conn);
 
+    // Extract @mentions from comment text
+    $mentions = $commentModel->extractMentions($data['comment_text'] ?? '');
+    $mentionedUsers = [];
+    if (!empty($mentions)) {
+        $mentionedUsers = $commentModel->getMentionedUsers($mentions);
+    }
+
     // Add comment with user tracking
     $result = $commentModel->addComment($ticketId, $data, $userId);
 
     // Log comment creation to audit log
     if ($result['success'] && isset($result['comment_id'])) {
         $auditLog->logCommentCreate($userId, $result['comment_id'], $ticketId);
+
+        // Log mentions to audit log
+        foreach ($mentionedUsers as $mentionedUser) {
+            $auditLog->log(
+                $userId,
+                'mention',
+                'user',
+                (string)$mentionedUser['user_id'],
+                [
+                    'ticket_id' => $ticketId,
+                    'comment_id' => $result['comment_id'],
+                    'mentioned_username' => $mentionedUser['username']
+                ]
+            );
+        }
+
+        // Add mentioned users to result for frontend
+        $result['mentions'] = array_map(function($u) {
+            return $u['username'];
+        }, $mentionedUsers);
     }
 
     // Add user display name to result for frontend