Organize backend into product executable and shared Go modules

backend/shared/README.md

@@ -0,0 +1,12 @@
+# Shared backend library
+
+This is a Go library module, not an executable. It provides database connections,
+transactions and repository primitives, errors, structured logging, generic HTTP
+responses, configuration and rate-limit helpers. It contains no customer, session,
+verification or Sendico domain models and imports no executable SXCHNGE module.
+
+`product` consumes it through a local module replacement. A future backoffice
+executable can consume the same infrastructure independently.
+
+Run `go test -race ./...` and `go vet ./...` here, or use the root Make targets to
+validate all implemented backend modules.