Self-host: rebuild the static site automatically on publish (follow-up to #143) #150

Context

#143 / PR #148 ship the self-host Docker image with site serving option (a): cloudhal build writes static HTML to /data/dist and Caddy serves it from the volume. The image exposes this as the site-build command (docker compose exec cloudhal site-build), run by hand or from cron after a publish. Nothing rebuilds the site automatically when an editor or an agent publishes, so a self-hosted site can serve stale pages until someone runs the command.

Expected

A publish on a self-hosted installation refreshes the served site without an operator step:

• A publish hook on the Node server (cloudhal studio): when a snapshot is published (the publish path in @cloudhal/core-ts / the API's publish route and RFC-0008's scheduled releases), schedule one cloudhal build run in a child process, debounced so a burst of publishes builds once, serialised so two builds never overlap, with the result (duration, snapshot id, failure) written to the audit log and visible in Studio.
• The purge step that the Cloudflare site Worker has (onPublished + the release purger) is the natural seam; self-host should hang the build on the same event, so the two runtimes stay symmetrical.
• Failure handling: a failed build keeps the previous dist (build into a temporary directory, swap atomically), reports in Studio and retries on the next publish.
• A CLOUDHAL_SITE_BUILD=off escape hatch for installations that build elsewhere (CI, another host), and documentation in docs/guides/self-host.md replacing the manual site-build paragraph.

Not in scope

Live rendering of the site from the store on Node (an Astro Node adapter for starters/blog) is a separate RFC; this issue only closes the gap in option (a).