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/services/publish_service.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from app.repositories.audit import create_audit_event
 from app.repositories.scheme_version_pricing import replace_scheme_version_pricing_snapshot
 from app.repositories.scheme_versions import get_current_scheme_version
@@ -41,13 +43,20 @@ async def publish_current_draft_scheme(
     readiness = await build_publish_readiness(
         scheme_id=scheme.scheme_id,
         scheme_version_id=version.scheme_version_id,
+        status=version.status,
     )
 
     if not readiness["readiness"]["is_ready_to_publish"]:
         raise_conflict(
             code="publish_not_ready",
-            message="Draft scheme does not satisfy publish readiness requirements.",
-            details={"readiness": readiness["readiness"]},
+            message="Scheme is not ready to publish in current draft state.",
+            details={
+                "scheme_version_id": version.scheme_version_id,
+                "readiness": readiness["readiness"],
+                "validation_summary": readiness["validation_summary"],
+                "pricing_coverage": readiness["pricing_coverage"],
+                "snapshot": readiness["snapshot"],
+            },
         )
 
     snapshot = await replace_scheme_version_pricing_snapshot(