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/services/draft_guard.py

@@ -1,3 +1,5 @@
+from fastapi import HTTPException, status
+
 from app.repositories.scheme_versions import get_current_scheme_version
 from app.repositories.schemes import get_scheme_record_by_scheme_id
 from app.services.api_errors import raise_conflict
@@ -27,19 +29,20 @@ async def get_current_draft_context(
     )
 
     if version.status != "draft" or scheme.status != "draft":
-        raise_conflict(
-            code="draft_not_editable",
-            message="Current scheme version is not editable because it is not in draft state",
-            scheme_status=scheme.status,
-            scheme_version_status=version.status,
-            actual_scheme_version_id=version.scheme_version_id,
+        raise HTTPException(
+            status_code=status.HTTP_409_CONFLICT,
+            detail="Current scheme version is not editable because it is not in draft state",
         )
 
     if expected_scheme_version_id and expected_scheme_version_id != version.scheme_version_id:
-        raise_conflict(**build_stale_draft_version_detail(
-            expected_scheme_version_id=expected_scheme_version_id,
-            actual_scheme_version_id=version.scheme_version_id,
-        ))
+        raise_conflict(
+            code="stale_draft_version",
+            message="Draft scheme version is stale. Reload current draft state before applying mutation.",
+            details={
+                "expected_scheme_version_id": expected_scheme_version_id,
+                "actual_scheme_version_id": version.scheme_version_id,
+            },
+        )
 
     return scheme, version