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/assign_ticket.php

@@ -1,8 +1,13 @@
 <?php
+// Apply rate limiting
+require_once dirname(__DIR__) . '/middleware/RateLimitMiddleware.php';
+RateLimitMiddleware::apply('api');
+
 session_start();
 require_once dirname(__DIR__) . '/config/config.php';
 require_once dirname(__DIR__) . '/models/TicketModel.php';
 require_once dirname(__DIR__) . '/models/AuditLogModel.php';
+require_once dirname(__DIR__) . '/models/UserModel.php';
 
 header('Content-Type: application/json');
 
@@ -50,6 +55,7 @@ if ($conn->connect_error) {
 
 $ticketModel = new TicketModel($conn);
 $auditLogModel = new AuditLogModel($conn);
+$userModel = new UserModel($conn);
 
 if ($assignedTo === null || $assignedTo === '') {
     // Unassign ticket
@@ -58,6 +64,15 @@ if ($assignedTo === null || $assignedTo === '') {
         $auditLogModel->log($userId, 'unassign', 'ticket', $ticketId);
     }
 } else {
+    // Validate assigned_to is a valid user ID
+    $assignedTo = (int)$assignedTo;
+    $targetUser = $userModel->getUserById($assignedTo);
+    if (!$targetUser) {
+        echo json_encode(['success' => false, 'error' => 'Invalid user ID']);
+        $conn->close();
+        exit;
+    }
+
     // Assign ticket
     $success = $ticketModel->assignTicket($ticketId, $assignedTo, $userId);
     if ($success) {