Integrate email notifications into signup and login (#38)

backend/notifications/README.md

@@ -40,7 +40,8 @@ Multiple service workers can safely compete for jobs through atomic claims.
 
 ## Templates and parameters
 
-See `templates.example.json`. Parameters use Go template map syntax such as
+See `templates.example.json`, including the `auth.activation` and `auth.login`
+English/Russian templates used by [product authentication](../../docs/auth-notifications.md). Parameters use Go template map syntax such as
 `{{.first_name}}`. Each revision declares the exact allowed parameter names and
 locale-specific content. Missing/extra parameters, unknown revisions/locales and
 invalid content are rejected before acceptance. Empty locale uses the default.