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_library.py

@@ -40,5 +40,5 @@ async def test_create_library(
         assert result["name"] == "Test Library"
         assert result["root_path"] == f"{tmp_path}/books"
         assert result["path_template"] == "{author}/{title}"
-        assert result["read_only"] == False
+        assert result["read_only"] is False
         assert result["description"] is None