Function reinit_backend 

pub async fn reinit_backend<B: Backend>(
    project: &Project,
) -> Result<B, FailToInitBackend>

Re-initialize a backend, preserving cache directories.

This function:

1. Identifies cache paths that should be preserved (from Backend::CACHE_PATHS)
2. Deletes all non-cache items in the backend directory
3. Calls Backend::init() to re-scaffold the backend

This allows template updates to be applied while keeping expensive build caches.

Errors

Returns an error if the backend directory cannot be read, cleaned, or re-initialized.