Stream a GCS file as a binary CSV attachment

Retrieves a file from the configured Google Cloud Storage bucket by its object path and streams it back to the caller as a binary attachment. The response carries a Content-Disposition: attachment; filename="data.csv" header, which triggers a browser download using the static filename data.csv regardless of the original object name. Use this endpoint when you need the raw file bytes, such as offering the original CSV for download; use GET /file/read instead when you need the file parsed into structured JSON. Supply the path query parameter with the GCS object name (for example, imports/my-file.csv) and a valid JWT. On success returns HTTP 200 with the file content as text/csv.