Suggestion - Add API for files in processing queue by checksum

When uploading a bunch of (multiple) files to docspell, you may use the API "checkfile-checksum-by-id" to see, if a document is already (processed) in docspell.
However during the upload, there may files already in queue for processing.
So the API "checkfile-checksum-by-id" returns false and a possible duplicate document (which is already processing or in queue),
can be uploaded as long as the item is not completely processed.

Checking a checksum beforehand (which is in the queue/processed), can prevent to upload the same file multiple times.