feat(backend): add publish readiness contract and pricing diagnostics

add backend readiness contract for publish prechecks

add pricing diagnostics to explain publish-blocking conditions
make publish decisions more explicit and easier to debug for clients

backend/app/api/routes/publish.py

@@ -67,25 +67,16 @@ async def get_draft_publish_readiness(
     readiness = await build_publish_readiness(
         scheme_id=scheme.scheme_id,
         scheme_version_id=version.scheme_version_id,
-        require_full_pricing_coverage=settings.publish_require_full_pricing_coverage,
-    )
-    return PublishReadinessResponse(
-        scheme_id=scheme.scheme_id,
-        scheme_version_id=version.scheme_version_id,
-        status=version.status,
-        validation_summary=readiness["validation_summary"],
-        pricing_coverage=readiness["pricing_coverage"],
-        snapshot=readiness["snapshot"],
-        readiness=readiness["readiness"],
     )
+    return PublishReadinessResponse(**readiness)
 
 
 @router.get(f"{settings.api_v1_prefix}/schemes/{{scheme_id}}/draft/publish-preview", response_model=PublishPreviewResponse)
 async def get_publish_preview(
     scheme_id: str,
     baseline_scheme_version_id: str | None = Query(default=None),
-    refresh: bool = Query(default=False),
     expected_scheme_version_id: str | None = Query(default=None),
+    refresh: bool = Query(default=False),
     role: str = Depends(require_api_key),
 ):
     scheme, version = await get_current_draft_context(