feat(backend): add publish readiness endpoint and enforce publish gate contract

add backend endpoint for publish readiness checks

enforce publish gate contract before version publication
make publish preconditions explicit and consistent for clients

backend/docs/api-map.md

@@ -89,3 +89,6 @@
 - This file is an operational route index, not a generated OpenAPI export.
 - Update this map in the same change set when adding, removing, renaming, or moving routes.
 - Query guards such as expected_current_scheme_version_id / expected_scheme_version_id are part of the operational contract for optimistic concurrency on mutable flows.
+
+## app/api/routes/publish.py
+- GET /api/v1/schemes/{scheme_id}/draft/publish-readiness