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/015_ticket_dependencies.sql

@@ -0,0 +1,15 @@
+-- Migration: Create ticket dependencies table
+-- Version: 015
+
+CREATE TABLE IF NOT EXISTS ticket_dependencies (
+    dependency_id INT AUTO_INCREMENT PRIMARY KEY,
+    ticket_id VARCHAR(9) NOT NULL,
+    depends_on_id VARCHAR(9) NOT NULL,
+    dependency_type ENUM('blocks', 'blocked_by', 'relates_to', 'duplicates') DEFAULT 'blocks',
+    created_by INT NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE KEY unique_dependency (ticket_id, depends_on_id, dependency_type),
+    INDEX idx_ticket_id (ticket_id),
+    INDEX idx_depends_on_id (depends_on_id),
+    FOREIGN KEY (created_by) REFERENCES users(user_id) ON DELETE SET NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;