Create or upgrade a synced link

Standing promise: backfill now, then stay current (the sync sweeper re-runs synced links on a schedule). Direction both creates paired import/export links and defaults export to a CaseMark Output subfolder. Same body as /transfer minus run_mode. Upserts links by (connection_id, direction, remote, vault_id); existing once-links are upgraded in place with their ledger and cursor preserved. Downgrade or pause via PATCH /links/.