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/017_recurring_tickets.sql

@@ -0,0 +1,29 @@
+-- Migration: Create recurring tickets table
+-- Description: Enables automatic ticket creation on a schedule
+
+CREATE TABLE IF NOT EXISTS recurring_tickets (
+    recurring_id INT AUTO_INCREMENT PRIMARY KEY,
+    title_template VARCHAR(255) NOT NULL,
+    description_template TEXT,
+    category VARCHAR(50) DEFAULT 'General',
+    type VARCHAR(50) DEFAULT 'Task',
+    priority INT DEFAULT 4,
+    assigned_to INT NULL,
+    schedule_type ENUM('daily', 'weekly', 'monthly') NOT NULL,
+    schedule_day INT NULL COMMENT 'Day of week (1-7) for weekly, day of month (1-31) for monthly',
+    schedule_time TIME DEFAULT '09:00:00',
+    next_run_at TIMESTAMP NOT NULL,
+    last_run_at TIMESTAMP NULL,
+    is_active BOOLEAN DEFAULT TRUE,
+    created_by INT NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+    FOREIGN KEY (assigned_to) REFERENCES users(user_id) ON DELETE SET NULL,
+    FOREIGN KEY (created_by) REFERENCES users(user_id) ON DELETE SET NULL,
+    INDEX idx_recurring_next_run (next_run_at, is_active),
+    INDEX idx_recurring_active (is_active)
+);
+
+-- Sample recurring ticket for testing (commented out)
+-- INSERT INTO recurring_tickets (title_template, description_template, category, type, schedule_type, schedule_day, next_run_at)
+-- VALUES ('Weekly Server Maintenance Check', 'Perform weekly server health check and maintenance tasks.', 'Maintenance', 'Task', 'weekly', 1, NOW());