chore: bring the test suite up to ruff's standards

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.

backend/tests/integration/test_bookshelf.py

@@ -1,4 +1,3 @@
-import pytest
 from httpx import AsyncClient
 
 
@@ -200,7 +199,6 @@ async def test_remove_books_from_shelf(
         "/books", params={"shelves": shelf_id}
     )
 
-    
     assert books_response.status_code == 200
     assert books_response.json()["total"] == 2