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/routes/api/admin_sites.py

@@ -75,6 +75,7 @@ class BuildPocRequest(BaseModel):
     prospect_id: int
     template_id: str
     merchant_id: int | None = None
+    site_id: int | None = None  # If set, populate existing site instead of creating new one
 
 
 class BuildPocResponse(BaseModel):
@@ -100,6 +101,7 @@ def build_poc(
         prospect_id=data.prospect_id,
         template_id=data.template_id,
         merchant_id=data.merchant_id,
+        site_id=data.site_id,
     )
     db.commit()
     return BuildPocResponse(**result)