Move a sprint item to another version, keeping its status and title.

Body: {"next": true} (the server computes the version following the item's own and returns it) or {"to_version": "v2.5"}; optionally expected_version (the version the caller saw -- a mismatch is a 409, which keeps a replayed next from moving the item twice). Unlike /push the item is NOT deferred: it stays pending and shows up under the target version. Response: {item, from_version, to_version, via, unchanged, moved_children, history_recorded}.