feat: implement group-based access control (Admin, Standard_User, Leadership, Read_Only)
- Add user_group migration and created_by column migration - Replace requireRole middleware with requireGroup - Update all backend routes to use group-based authorization - Add Standard_User conditional delete with ownership, state, and compliance checks - Add cascade impact check for CVE deletes - Update AuthContext with group-based permission helpers - Update all frontend components for group-based rendering - Update UserManagement UI with group dropdown, confirmation dialogs, self-demotion prevention
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