Use platform-native file change APIs to track which stored-file
attachments have been modified, avoiding expensive full scans of all
attachment files during sync.
Backends:
- macOS: FSEvents (persistent event journal, survives restarts)
- Windows: ReadDirectoryChangesW (live recursive directory watch
via overlapped I/O polling)
- Linux: inotify (live per-directory watches)
On unsupported platforms or on error, falls back to existing scan logic.
