Export Workspace To Path Endpoint

Write a workspace's .qualia archive into a local directory.

The download endpoint hands the archive to the browser, which cannot survive a multi-gigabyte workspace export: buffering it crashes the renderer and takes the app's socket connection with it. Here the backend writes the file itself, so the bytes never leave this process. Only available when the workspace is on this machine — anywhere else the path would name a directory the user cannot see.