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>

uploads/.htaccess

@@ -0,0 +1,30 @@
+# Deny direct access to uploaded files
+# All downloads must go through download_attachment.php
+
+<IfModule mod_authz_core.c>
+    Require all denied
+</IfModule>
+
+<IfModule !mod_authz_core.c>
+    Order deny,allow
+    Deny from all
+</IfModule>
+
+# Disable script execution
+<IfModule mod_php.c>
+    php_flag engine off
+</IfModule>
+
+# Prevent directory listing
+Options -Indexes
+
+# Block common executable extensions
+<FilesMatch "\.(php|phtml|php3|php4|php5|php7|phps|phar|cgi|pl|py|sh|bash|exe|com|bat|cmd|vbs|js|html|htm|asp|aspx|jsp)$">
+    <IfModule mod_authz_core.c>
+        Require all denied
+    </IfModule>
+    <IfModule !mod_authz_core.c>
+        Order deny,allow
+        Deny from all
+    </IfModule>
+</FilesMatch>