- Limit to one on-demand crawl per path. Don't start another if one is already running. See HandleFileNotFound()
- Add config value to limit the number of on-demand crawls
- Add config value to limit the number of concurrent crawls, other crawls get queued.


Later:
- Add a wildcard option to restricted_download_paths to block all sub-directories
- Add a dict to each restricted_download_paths item to specify how many levels recursive the block should be applied
- add a "last modified" to "sort" https://chub-archive.evulid.cc/api/file/list?path=/chub.ai/characters&page=1&limit=50&sort=folders
- add an admin endpoint to fetch the last n modified files.