Show expired artifacts instead of an access spinner

[none23]

Summary

Show authorized viewers that an artifact has expired instead of leaving the shared-access check spinning indefinitely.

Changes

• Return distinct accepted, expired, and unavailable access outcomes while preserving artifact privacy.
• Render an expiration screen with the recorded expiration time and republishing guidance.
• Fall back to the unavailable state if a successful access handshake does not refresh the artifact query.
• Isolate transient access state per artifact URL so navigation cannot reuse stale outcomes.