fix(backend): make archive and unarchive timestamps monotonic and auditable

[arisu6804]

Problem

Repeated archive/unarchive operations can overwrite timestamps or hide the true lifecycle order, complicating reconciliation and retention.

Objective

Deliver a production-quality improvement to transfer archival lifecycle and audit records that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Define monotonic state transitions and immutable event timestamps; reject stale commands; record actor and reason for each change.

Acceptance criteria

• Current state and history remain consistent under retries and concurrency; timestamps never move backward; each transition is auditable.

Required validation

• State-machine, stale-command, race, retry, and event-order tests.
• Existing tests and CI remain passing.
• Add regression coverage for the original failure mode.
• Do not weaken, delete, or skip unrelated tests to obtain a green build.

PR quality bar

• Keep the PR focused and explain design tradeoffs, compatibility impact, and test evidence.
• Avoid typo-only, documentation-only, cosmetic-only, or unrelated refactor submissions.

Out of scope

• Broad rewrites not required by the acceptance criteria.
• Changes to unrelated services, contracts, or user flows.