Expire artifacts automatically to protect workspace quota - #5

Summary

Expire artifacts after three days by default so stale HTML is reclaimed before it blocks new publishes. Every republish resets the timer unless the owner explicitly chooses another duration or disables expiration.

Changes

• Store and enforce one server-owned expiration timestamp across list, view, invitation, deletion, and capacity paths.
• Reclaim expired artifact chunks, grants, and metadata before publish quota checks and when owners open the library.
• Add browser controls for one-hour through 30-day lifetimes and never-expiring artifacts.
• Add --expires-in to the publisher and skill, plus expiresInSeconds / expiresAt to the API contract.
• Preserve pre-existing artifacts as non-expiring until their next update and document Lakebed's lazy physical cleanup behavior.

Demo

Artifact expiration solution brief (private to configured owners and workspace viewers)