Snapshot the live WAL to a timestamped copy — fast (tens of MB), no big copy.

WAL-ONLY by design (ops choice 2026-06-13): preserves recent un-checkpointed updates before a risky op or restart. IMPORTANT caveat surfaced in the response: a WAL snapshot is only restorable PAIRED with the main DB file as it stood at snapshot time, so we record the main file's size + mtime as the pairing reference. If the main file is later checkpointed/changed, the snapshot may no longer apply cleanly — for a fully self-sufficient backup use /api/admin/backup-db (full rotation) instead.