Integrate email notifications into signup and login (#38)

backend/product/README.md

@@ -24,8 +24,11 @@ organization, role, policy or seed; the runtime creates its indexes.
 The runtime mounts health, login, API login, refresh, rotation and logout routes.
 Customer/application-client records must exist for login. The customer service
 supports atomic account/activation creation and deletion with session revocation.
-Signup/profile HTTP wiring and verification delivery/completion remain follow-up
-work. The frontend still uses dummy services.
+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.
 
 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;