Add cluster backup, restore, and failure runbook

[TheSilkky]

Closes #97

Target base branch

• Base: develop
• Head: issue-97-add-cluster-backup-restore-and-failure-runbooks

Summary

• Add a cluster backup, restore, and failure runbook for optional PostgreSQL metadata, S3-compatible encrypted blobs, and Valkey/Redis-compatible coordination.
• Link the runbook from README, docs index, deployment, configuration, production-cluster scope, retention, security model, threat model, and code map docs.
• Preserve existing warnings that Proofline is not production-ready public infrastructure and that /v1, key custody, and backend decryption boundaries are unchanged.

Docs updated

• docs/cluster-backup-restore-runbook.md
• README.md
• docs/README.md
• docs/deployment.md
• docs/production-cluster-scope.md
• docs/retention-backup-deletion.md
• docs/configuration.md
• docs/security-model.md
• docs/threat-model.md
• docs/code-map.md
• CHANGELOG.md

Validation

• git diff --check
• git diff --no-index --check /dev/null docs/cluster-backup-restore-runbook.md
• Markdown link-target check for changed docs
• ASCII check for changed docs
• Sensitive-example scan for changed docs
• go test ./... skipped; documentation-only change
• go vet ./... skipped; documentation-only change

Revalidation

• Issue Add Cluster Backup Restore And Failure Runbooks #97 was drafted from develop at 539bb8d0a2cab1b9dc5e18ff394ae9959c8d7350.
• Revalidated against current origin/develop at dee2abbd7e61a2bc865b103e86d81a3deca79c4a before creating this PR.

Key custody / decryption

• No key custody, backend decryption, key escrow, browser decryption, or raw-key handling changes.
• The runbook is operational guidance only and does not claim production readiness.