Backup / restore + disaster-recovery runbook

## Context
Operational safety for self-hosters and the hosted offering.

## Scope
- [ ] Postgres backup/restore procedure + object-store backup
- [ ] Point-in-time recovery guidance
- [ ] DR runbook with RPO/RTO targets
- [ ] Restore drill validated in CI/staging

## Acceptance criteria
- A documented, tested path to restore the platform from backup