No backup/restore story for the note database

Severity: high (for the stated production goal).

### Summary

There is no backup mechanism anywhere: no `VACUUM INTO`/`.backup`, no litestream/WAL-shipping sidecar in `bin/node/docker/docker-compose.yml` (only telemetry services), nothing in the `Makefile` (only `docker volume create node-db`), nothing in the docs.

### Impact

For private notes, this DB is the only copy of `NoteDetails` before consumption (per #101's own framing). A lost volume is unrecoverable, fund-relevant data loss — and per the fail-open issue (#124) the service silently restarts empty rather than failing.

### Recommendation

- Minimum viable: periodic `VACUUM INTO '/backup/…'` (safe online under WAL) from the maintenance loop or a cron sidecar, plus documented restore steps.
- Better: litestream or WAL-shipping.
- Note the overlap with #111 — a Postgres backend inherits a mature backup ecosystem.

Related: #111, #112.

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Part of #114.