fix: integration fixes from final review

- useSettings: upsert instead of update (safe if migration rows missing)
- invite-admin-user: return created_at and last_sign_in_at in response
- UserManagementPage: clear deleteError when opening new confirmation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/admin/users/UserManagementPage.jsx

@@ -244,7 +244,7 @@ const UserManagementPage = () => {
                         </span>
                       ) : (
                         <button
-                          onClick={() => setConfirmDeleteId(user.id)}
+                          onClick={() => { setConfirmDeleteId(user.id); setDeleteError(''); }}
                           title="Удалить пользователя"
                           style={{
                             background: 'transparent',