File sharing session

[anna-parker]

Partially resolves #3935, resolves #4056

I started going through the file sharing code with claude and tackling some open issues

Screenshot

TODOs:

• ensure files submitted by the prepro pipeline are also marked as submitted and are not accidentally garbage collected (unless the pipeline submission fails and a redo is required) -> basically we need to update the code to check if a file ID is referenced in the unprocessed or processed data
• add garbage collection for old versions of processed files

PR Checklist

• All necessary documentation has been adapted.
• The implemented feature is covered by appropriate, automated tests.
• Any manual testing that has been done is documented (i.e. what exactly was tested?)

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[claude]

This PR may be related to: #4221 (File sharing)