Connect sign-up to Sendico Facts intake (#46)

backend/product/README.md

@@ -33,9 +33,10 @@ API versions are installed explicitly by the product composition. See
 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;
-copy only the wire definitions consumed by an implemented adapter.
+Optional `api.sendico` connects sign-up to the public individual registry and
+immutable Facts intake. The adapter owns organization context and consumed wire
+contracts; local customers retain no platform tenancy. See
+[configuration, recovery and measurement](../../docs/signup-facts.md).
 
 ## Validate