Document deletion-state restore drills

[TheSilkky]

Closes #201

Target base branch

• Base: develop
• Head: issue-201-restore-deletion-drill-docs

Summary

• Add private restore-drill validation steps for active, deleting, deletion-failed, deleted, and tombstone-pruned incident states.
• Clarify public viewer fail-closed checks after restore, including metadata/blob mismatch and deleted-state cases.
• Add sharing-grant and wrapped-key consistency checks for restored incidents and completed deletion pruning.

Validation

• git diff --check

Docs

• Updated cluster backup/restore, retention/backup/deletion, incident deletion enforcement, deployment checklist, and changelog docs.

Scope and safety

• Revalidated against develop.
• Docs-only change.
• No backup automation, cloud provider scripts, Kubernetes, Terraform, production-readiness claims, backend decryption, key escrow, or public route expansion.

Draft status

• Opened ready for review per maintainer open-PR override.