Mostly automatic: empty f-strings, unused imports, formatting. The manual half
was duplicated import blocks stranded mid-file and `== None` assertions, which
become `is None` here because they compare plain attributes -- unlike the
identical rule in filters/book.py, where they build SQL.
Two unused variables are the tests never checking the disk in either
delete_files direction. Left in place under a noqa so the gap stays visible.
patrick