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/services/poc_builder_service.py

@@ -35,9 +35,13 @@ class PocBuilderService:
         prospect_id: int,
         template_id: str,
         merchant_id: int | None = None,
+        site_id: int | None = None,
     ) -> dict:
         """Build a complete POC site from prospect data and a template.
 
+        If site_id is given, populates the existing site's store with CMS
+        content. Otherwise creates a new HostedSite + Store.
+
         Returns dict with hosted_site, store, pages_created, theme_applied.
         """
         from app.modules.prospecting.models import Prospect
@@ -57,18 +61,20 @@ class PocBuilderService:
         # 3. Build placeholder context from prospect data
         context = self._build_context(prospect)
 
-        # 4. Create HostedSite + Store
-        site_data = {
-            "business_name": context["business_name"],
-            "domain_name": prospect.domain_name,  # used for clean subdomain slug
-            "prospect_id": prospect_id,
-            "contact_email": context.get("email"),
-            "contact_phone": context.get("phone"),
-        }
-        if merchant_id:
-            site_data["merchant_id"] = merchant_id
-
-        site = hosted_site_service.create(db, site_data)
+        # 4. Use existing site or create new one
+        if site_id:
+            site = hosted_site_service.get_by_id(db, site_id)
+        else:
+            site_data = {
+                "business_name": context["business_name"],
+                "domain_name": prospect.domain_name,
+                "prospect_id": prospect_id,
+                "contact_email": context.get("email"),
+                "contact_phone": context.get("phone"),
+            }
+            if merchant_id:
+                site_data["merchant_id"] = merchant_id
+            site = hosted_site_service.create(db, site_data)
 
         # 5. Get the hosting platform_id from the store
         from app.modules.tenancy.models import StorePlatform