feat: production routing support for subdomain and custom domain modes

Double-mount store routes at /store/* and /store/{store_code}/* so the
same handlers work in dev path-based, prod path-based, prod subdomain,
and prod custom-domain modes.  Wire StorePlatform.custom_subdomain into
StoreContextMiddleware for per-platform subdomain overrides.  Add admin
custom-domain management UI, fix stale /shop/ reset link, add
/merchants/ to reserved paths, and server-render window.STORE_CODE for
JS that previously parsed the URL.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/customers/routes/api/storefront.py

@@ -262,7 +262,7 @@ def forgot_password(request: Request, email: str, db: Session = Depends(get_db))
 
             scheme = "https" if should_use_secure_cookies() else "http"
             host = request.headers.get("host", "localhost")
-            reset_link = f"{scheme}://{host}/shop/account/reset-password?token={plaintext_token}"
+            reset_link = f"{scheme}://{host}/account/reset-password?token={plaintext_token}"
 
             email_service = EmailService(db)
             email_service.send_template(