* Refactor Admin UI to use unified IAM storage and add MultipleFileStore
* Address PR feedback: fix renames, error handling, and sync logic in FilerMultipleStore
* Address refined PR feedback: safe rename order, rollback logic, and structural sync refinement
* Optimize LoadConfiguration: use streaming callback for memory efficiency
* Refactor UpdateUser: log rollback failures during rename
* Implement PolicyManager for FilerMultipleStore
* include the filer_multiple backend configuration
* Implement cross-S3 synchronization and proper shutdown for all IAM backends
* Extract Admin UI refactoring to a separate PR