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>

create_ticket_api.php

@@ -2,8 +2,7 @@
 header('Content-Type: application/json');
 
 error_reporting(E_ALL);
-ini_set('display_errors', 1);
-file_put_contents('debug.log', print_r($_POST, true) . "\n" . file_get_contents('php://input') . "\n", FILE_APPEND);
+ini_set('display_errors', 0);
 
 // Load environment variables with error check
 $envFile = __DIR__ . '/.env';
@@ -64,7 +63,6 @@ try {
 }
 
 $userId = $systemUser['user_id'];
-file_put_contents('debug.log', "Authenticated as system user ID: $userId\n", FILE_APPEND);
 
 // Create tickets table with hash column if not exists
 $createTableSQL = "CREATE TABLE IF NOT EXISTS tickets (