Parallelize `list_files_for_scan`

### Is your feature request related to a problem or challenge?

Cold runs are slow: due to reading / processing everything on one thread.

Currently `list_files_for_scan` runs in a single thread.

### Describe the solution you'd like

We should be able to use all threads.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_