handle_list_all_sessions returned client_mgr.list_sessions(), which
iterates user_clients_map across ALL users and returns every session's
StorageToken (token, client_url, machine_id, user_id). The handler is
mounted under login_required! but performed no per-user authorization:
it fetched get_group_permissions() only to println! the result, then
returned the full cross-user list. Any authenticated user could read
every other user's device token and public client_url.
Scope the result to the caller by adding
Storage::list_user_client_tokens(user_id) /
ClientManager::list_sessions_by_user_id(user_id), mirroring the existing
per-user pattern in handle_get_summary (list_machine_by_user_id). Also
drop the leftover debug println! and the unwrap() on the current user
(return 401 instead).
