The Build POC button on site detail now passes site_id to the POC
builder, which populates the existing site's store with CMS content
instead of trying to create a new site (which failed with duplicate
slug error).
- poc_builder_service.build_poc() accepts optional site_id param
- If site_id given: uses existing site, skips hosted_site_service.create()
- If not given: creates new site (standalone POC build)
- API schema: added site_id to BuildPocRequest
- Frontend: passes this.site.id in the build request
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>