Implement artefact storage abstraction, retention policy model, and encrypted secret-storage plumbing

[kalen-releasedgroup]

Summary

Prepare the application to store artefacts and sensitive material safely from the beginning.

Scope

• Implement the artefact storage abstraction and artefact record model.
• Implement retention policy configuration model aligned to Section 7.4.
• Add encrypted-at-rest handling for storage state, cookies, and sensitive variables.

Requirement Links

• Sections 7.4, 11.2, 12.2, 12.3, 12.5, 16.4
• FR-ADM-004

Dependencies

• Persistence baseline
• Security/key-management direction

Exit Criteria

• Artefacts can be stored behind a defined abstraction.
• Sensitive auth/session material is encrypted at rest.
• Retention policy metadata is persisted and available to later cleanup jobs.