Make product API versions independently installable

backend/product/README.md

@@ -28,7 +28,10 @@ Signup and email verification delivery/completion are available when
 `api.notifications` is configured. Flutter selects the backend using
 `SXCHNGE_API_URL` and `SXCHNGE_CLIENT_ID`. See
 [auth notification setup and contracts](../../docs/auth-notifications.md).
-Profile HTTP wiring remains follow-up work.
+API versions are installed explicitly by the product composition. See
+[version installation and compatibility](../../docs/api-versions.md) for selection,
+mounts, shared lifecycle and adding a future version. Customer profile endpoint
+wiring remains follow-up work.
 
 Sendico integration is not wired. Its organization context belongs only to the
 future public HTTP adapter. There is no unused public API snapshot in this module;