Manage the changelog archive and duplicate actions from the API and MCP

Changelog entries can now be unpublished — on a schedule or by hand — which archives them, and archived entries can be duplicated to publish a fresh copy. Both actions live in the admin today. The scheduled unpublish date is already available through the API, so an integration can set one when it creates or updates an entry. What is missing is the two actions themselves: there is no way to unpublish an existing entry, or duplicate one, without opening the admin. That leaves a gap for anyone automating their changelog — posting a maintenance notice from a deployment pipeline and taking it down again afterwards currently needs a person for the second half. The same applies to a connected AI agent: it can write and schedule an entry, but not retire one or reuse a past announcement. This covers exposing both actions through the public API and the Sleekplan MCP server, and letting archived entries be listed alongside drafts, scheduled and published ones.