Using jthread to solve .detach() problem on graceful shutdown

[hemant-endee]

PR Summary:

Problem: Backup thread used .detach() — on graceful shutdown, ~IndexManager destroyed members while the detached thread was still using them (use-after-free)

Solution :

• Using std::jthread (C++20). Thread handle stored in BackupStore, joined in destructor before members are destroyed
• jthread's built-in stop_token is passed into createBackupTar and checked per-file in the tar loop.
• On SIGTERM, the thread aborts within one file iteration instead of blocking shutdown until the entire tar completes