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/014_add_additional_indexes.sql

@@ -0,0 +1,23 @@
+-- Migration: Add additional indexes for improved query performance
+-- Version: 014
+
+-- Index for audit log queries by user and date (activity reports)
+CREATE INDEX IF NOT EXISTS idx_audit_log_user_created ON audit_log(user_id, created_at DESC);
+
+-- Index for audit log queries by action type (security monitoring)
+CREATE INDEX IF NOT EXISTS idx_audit_log_action_type ON audit_log(action_type, created_at DESC);
+
+-- Index for tickets by status only (status filtering)
+CREATE INDEX IF NOT EXISTS idx_tickets_status ON tickets(status);
+
+-- Composite index for common dashboard queries (status + priority + created_at)
+CREATE INDEX IF NOT EXISTS idx_tickets_status_priority_created ON tickets(status, priority, created_at DESC);
+
+-- Index for ticket comments by ticket_id and date (comment listing)
+CREATE INDEX IF NOT EXISTS idx_comments_ticket_created ON ticket_comments(ticket_id, created_at DESC);
+
+-- Index for API keys by key value (authentication lookups)
+CREATE INDEX IF NOT EXISTS idx_api_keys_key_value ON api_keys(key_value);
+
+-- Index for user preferences lookup
+CREATE INDEX IF NOT EXISTS idx_user_preferences_user_key ON user_preferences(user_id, preference_key);