v0.7.4 — fix app freeze during webpage archiving

Fixed

• Whole-app freeze while archiving a webpage. archive_website() is a blocking operation (network fetches, parallel prefetch, optional monolith/obelisk subprocess) but was being run directly on FastAPI's async event loop — both in the /api/archive route and in the +link note-save path. A slow or hung archive (common behind a corporate TLS-inspection proxy) blocked the event loop and made the entire app unresponsive, including page refreshes, until the archive finished or timed out. Both callsites now run the archive in a worker thread (asyncio.to_thread), so the UI stays responsive throughout.

Pairs with the v0.7.3 archive_ssl_verify toggle for archiving behind corporate proxies.