fix(hosting): cascade soft-delete Store when deleting HostedSite

When deleting a hosted site, the associated Store is now soft-deleted
(sets deleted_at). This frees the subdomain for reuse via the partial
unique index (WHERE deleted_at IS NULL).

Previously the Store was orphaned, blocking subdomain reuse.
Closes docs/proposals/hosting-cascade-delete.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/hosting/services/hosted_site_service.py

@@ -218,8 +218,19 @@ class HostedSiteService:
         return site
 
     def delete(self, db: Session, site_id: int) -> bool:
+        """Delete a hosted site and soft-delete the associated store."""
+        from app.core.soft_delete import soft_delete
+
         site = self.get_by_id(db, site_id)
+        store = site.store
+
         db.delete(site)
+
+        # Soft-delete the store created for this site (frees the subdomain)
+        if store:
+            soft_delete(db, store)
+            logger.info("Soft-deleted store %d (subdomain=%s) for site %d", store.id, store.subdomain, site_id)
+
         db.flush()
         logger.info("Deleted hosted site: %d", site_id)
         return True