feat(backend): add publish readiness contract and guarded publish flow

add backend readiness contract for publish prechecks

guard publish flow with explicit validation and version-aware checks
make publish behavior more predictable for clients and safer against stale state

backend/app/api/routes/schemes.py

@@ -18,7 +18,7 @@ from app.repositories.schemes import (
     rollback_scheme_to_version,
     unpublish_scheme,
 )
-from app.schemas.publish_readiness import SchemePublishActionResponse
+from app.schemas.publish_readiness import PublishExecutionResponse
 from app.schemas.scheme_registry import (
     SchemeCurrentResponse,
     SchemeDetailResponse,
@@ -217,17 +217,18 @@ async def get_publish_validation(scheme_id: str, role: str = Depends(require_api
 
 @router.post(
     f"{settings.api_v1_prefix}/schemes/{{scheme_id}}/publish",
-    response_model=SchemePublishActionResponse,
+    response_model=PublishExecutionResponse,
 )
 async def publish_scheme_endpoint(
     scheme_id: str,
     expected_scheme_version_id: str | None = Query(default=None),
     role: str = Depends(require_api_key),
 ):
-    return await publish_current_draft_scheme(
+    result = await publish_current_draft_scheme(
         scheme_id=scheme_id,
         expected_scheme_version_id=expected_scheme_version_id,
     )
+    return PublishExecutionResponse(**result)
 
 
 @router.post(f"{settings.api_v1_prefix}/schemes/{{scheme_id}}/unpublish", response_model=SchemePublishResponse)