Backup API and the scribase-backup tool, end to end

The durable backup path is complete: a dedicated operator tool plus the /v1 backup routes.

• The scribase-backup operator binary ships create, upload (to S3-compatible object storage under content-addressed keys), and prepare-restore-target. Backup records live in durable Postgres-backed state; list, create, and restore them through the /v1 API or the console.
• Backups use PostgreSQL custom format, validate via pg_restore --list, compute SHA-256 evidence, and publish archive and manifest atomically with no-clobber links.
• GET/POST /v1/…/environments/{id}/backups and POST /v1/…/backups/{id}/restore are live and contract-tested against the OpenAPI spec.
• Every API mutation takes an idempotency key and returns an operation ID.