Archives an existing evidence request and records audit metadata.

This endpoint is typically used when:

• A reviewer or system wants to mark an evidence request as archived
• Workflows require closing outdated or completed requests
• Auditing requires capturing who performed the archival action

Behavior:

• If the request is already archived, the operation is a no-op but still returns HTTP 200.
• Audit metadata (actor, timestamp, etc.) is recorded for compliance.