fix(hosting): POC builder works with existing sites

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>

app/modules/hosting/templates/hosting/admin/site-detail.html

@@ -351,6 +351,7 @@ function hostingSiteDetail(siteId) {
                 var result = await apiClient.post('/admin/hosting/sites/poc/build', {
                     prospect_id: this.site.prospect_id,
                     template_id: this.selectedTemplate,
+                    site_id: this.site.id,
                 });
                 this.pocResult = 'POC built! ' + result.pages_created + ' pages created.';
                 Utils.showToast('POC built successfully', 'success');