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

backend/routes/auditLog.js

@@ -1,11 +1,11 @@
 // Audit Log Routes (Admin only)
 const express = require('express');
 
-function createAuditLogRouter(db, requireAuth, requireRole) {
+function createAuditLogRouter(db, requireAuth, requireGroup) {
     const router = express.Router();
 
-    // All routes require admin role
-    router.use(requireAuth(db), requireRole('admin'));
+    // All routes require Admin group
+    router.use(requireAuth(db), requireGroup('Admin'));
 
     // Get paginated audit logs with filters
     router.get('/', async (req, res) => {