Fixed three more cross-user WebSocket leaks in multi-user deployments (follow-up to the ``settings_changed`` fix): ``parallel_search_started`` — which carried the user's **raw research query** — and ``engine_completed`` were broadcast to every connected client despite having no frontend listener, and are removed entirely; ``search_engine_selected`` (shown in the research log panel) is now scoped to the owning user's per-user room, resolved from the research worker's search context, and is skipped entirely — never broadcast — when no username is available.
