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_calibre_import.py

@@ -42,7 +42,9 @@ def fx_source(tmp_path: Path) -> Path:
         cover=True,
         formats={"EPUB": EPUB},
     )
-    fixture.add_book(2, "The Art of War", authors=["Sun Tzu"], formats={"EPUB": OTHER_EPUB})
+    fixture.add_book(
+        2, "The Art of War", authors=["Sun Tzu"], formats={"EPUB": OTHER_EPUB}
+    )
     fixture.add_book(3, "Metadata Only", authors=["Nobody"])
 
     return fixture.commit()
@@ -95,7 +97,8 @@ async def test_an_uploaded_library_imports(
     authenticated_client: AsyncClient, source: Path, tmp_path: Path
 ) -> None:
     status, job = await upload(
-        authenticated_client, zip_of(source, tmp_path / "out", prefix="Calibre Library/")
+        authenticated_client,
+        zip_of(source, tmp_path / "out", prefix="Calibre Library/"),
     )
 
     assert status == 202
@@ -278,7 +281,9 @@ async def test_a_second_copy_is_counted_as_a_possible_duplicate(
     padded.write_bytes(EPUB.read_bytes() + b"\0" * 64)
 
     fixture = CalibreFixture(tmp_path / "calibre")
-    fixture.add_book(1, "The Metamorphosis", authors=["Franz Kafka"], formats={"EPUB": EPUB})
+    fixture.add_book(
+        1, "The Metamorphosis", authors=["Franz Kafka"], formats={"EPUB": EPUB}
+    )
     fixture.add_book(
         2, "The Metamorphosis", authors=["Franz Kafka"], formats={"EPUB": padded}
     )