User last activity helpers

See User_last_activity (database) for schema and RLS.

Each user has at most one row. Auth records it automatically on JWT issue / token refresh, at most once a day. Reads of another user's row require user_last_activity.read on a shared herd, or platform superadmin.

get_user_last_activity_by_user_id

• Args: client, user_id
• Returns: { status, msg, data: IUserLastActivity | null }