[feature/backend] implement /users handler + switch to username + add display name + user management cli

backend/internal/rbac/init_test.go

@@ -64,6 +64,7 @@ func TestAssignRoleToUser(t *testing.T) {
 	// Create a test user
 	user, err := client.User.Create().
 		SetEmail("test@example.com").
+		SetUsername("testuser").
 		SetPasswordHash("$2a$10$hzLdXMZEIzgr8eGXL0YoCOIIrQhqEj6N.S3.wY1Jx5.4vWm1ZyHyy").
 		Save(ctx)
 	if err != nil {