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>

models/CommentModel.php

@@ -1,10 +1,54 @@
 <?php
 class CommentModel {
     private $conn;
-    
+
     public function __construct($conn) {
         $this->conn = $conn;
     }
+
+    /**
+     * Extract @mentions from comment text
+     *
+     * @param string $text Comment text
+     * @return array Array of mentioned usernames
+     */
+    public function extractMentions($text) {
+        $mentions = [];
+        // Match @username patterns (alphanumeric, underscores, hyphens)
+        if (preg_match_all('/@([a-zA-Z0-9_-]+)/', $text, $matches)) {
+            $mentions = array_unique($matches[1]);
+        }
+        return $mentions;
+    }
+
+    /**
+     * Get user IDs for mentioned usernames
+     *
+     * @param array $usernames Array of usernames
+     * @return array Array of user records with user_id, username, display_name
+     */
+    public function getMentionedUsers($usernames) {
+        if (empty($usernames)) {
+            return [];
+        }
+
+        $placeholders = str_repeat('?,', count($usernames) - 1) . '?';
+        $sql = "SELECT user_id, username, display_name FROM users WHERE username IN ($placeholders)";
+        $stmt = $this->conn->prepare($sql);
+
+        $types = str_repeat('s', count($usernames));
+        $stmt->bind_param($types, ...$usernames);
+        $stmt->execute();
+        $result = $stmt->get_result();
+
+        $users = [];
+        while ($row = $result->fetch_assoc()) {
+            $users[] = $row;
+        }
+        $stmt->close();
+
+        return $users;
+    }
     
     public function getCommentsByTicketId($ticketId) {
         $sql = "SELECT tc.*, u.display_name, u.username