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>

migrations/016_ticket_attachments.sql

@@ -0,0 +1,18 @@
+-- Migration: Create ticket attachments table
+-- Date: 2026-01-19
+-- Description: Adds support for file attachments on tickets
+
+CREATE TABLE IF NOT EXISTS ticket_attachments (
+    attachment_id INT AUTO_INCREMENT PRIMARY KEY,
+    ticket_id VARCHAR(9) NOT NULL,
+    filename VARCHAR(255) NOT NULL,
+    original_filename VARCHAR(255) NOT NULL,
+    file_size INT NOT NULL,
+    mime_type VARCHAR(100) NOT NULL,
+    uploaded_by INT NULL,
+    uploaded_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    FOREIGN KEY (ticket_id) REFERENCES tickets(ticket_id) ON DELETE CASCADE,
+    FOREIGN KEY (uploaded_by) REFERENCES users(user_id) ON DELETE SET NULL,
+    INDEX idx_attachments_ticket (ticket_id),
+    INDEX idx_attachments_uploaded_by (uploaded_by)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;