Automate SQLite backups

[timpara]

Context

README documents a manual `VACUUM INTO` snapshot. For a continuously-deployed instance this should be automated.

Options

• Cron sidecar running `VACUUM INTO /backups/hours-$(date +%F).db` daily, retaining last N days. Simple, offline-friendly.
• Litestream sidecar streaming WAL to S3/B2/Azure Blob. Near-zero RPO, needs object storage.

Proposal

1. Ship a `deploy/docker-compose.backup.yml` overlay with a `litestream/litestream` sidecar and a `deploy/litestream.yml` template parameterised via env vars.
2. Also ship a minimal cron-based overlay for airgapped deployments.
3. Document restore procedure in README.