feat: add synchronous OLX export for legacy (v1) libraries (#38920)

Adds a library branch to export_handler so that GET /export/<library_key>
generates and streams the OLX .tar.gz directly to the browser. Unlike the
async Celery flow used for courses, libraries are small enough to export
synchronously in a single request with no need to queue a background task,
poll for status, or store the output file.

This supports the Export button added in the Authoring MFE migration wizard.
See: https://github.com/openedx/frontend-app-authoring/issues/2911

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>