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/auth.js

@@ -110,7 +110,7 @@ function createAuthRouter(db, logAudit) {
                 action: 'login',
                 entityType: 'auth',
                 entityId: null,
-                details: { role: user.role },
+                details: { group: user.user_group },
                 ipAddress: req.ip
             });
 
@@ -120,7 +120,7 @@ function createAuthRouter(db, logAudit) {
                     id: user.id,
                     username: user.username,
                     email: user.email,
-                    role: user.role
+                    group: user.user_group
                 }
             });
         } catch (err) {
@@ -183,7 +183,7 @@ function createAuthRouter(db, logAudit) {
         try {
             const session = await new Promise((resolve, reject) => {
                 db.get(
-                    `SELECT s.*, u.id as user_id, u.username, u.email, u.role, u.is_active
+                    `SELECT s.*, u.id as user_id, u.username, u.email, u.user_group, u.is_active
                      FROM sessions s
                      JOIN users u ON s.user_id = u.id
                      WHERE s.session_id = ? AND s.expires_at > datetime('now')`,
@@ -210,7 +210,7 @@ function createAuthRouter(db, logAudit) {
                     id: session.user_id,
                     username: session.username,
                     email: session.email,
-                    role: session.role
+                    group: session.user_group
                 }
             });
         } catch (err) {