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/unit/test_metadata_extractor.py

@@ -22,7 +22,6 @@ class TestEpubExtractor:
         assert metadata["published_date"] == date(year=2001, month=7, day=1)
 
 
-
 EPUB = Path("tests/data_files/Metamorphosis - Franz Kafka.epub")
 PDF = Path("tests/data_files/Calculus Made Easy - Silvanus Thompson.pdf")
 
@@ -31,7 +30,9 @@ PDF = Path("tests/data_files/Calculus Made Easy - Silvanus Thompson.pdf")
 class TestIdentifierMerging:
     """A book's formats each contribute identifiers; none of them replaces the rest."""
 
-    async def test_every_format_contributes(self, monkeypatch: pytest.MonkeyPatch) -> None:
+    async def test_every_format_contributes(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
         """
         Identifiers are a collection, not a single value.
 
@@ -47,7 +48,9 @@ class TestIdentifierMerging:
             }
 
         async def pdf(_file):
-            return {"identifiers": {"isbn-13": "9781718500402", "isbn-10": "1593270356"}}
+            return {
+                "identifiers": {"isbn-13": "9781718500402", "isbn-10": "1593270356"}
+            }
 
         monkeypatch.setattr(EpubExtractor, "extract_metadata", epub)
         monkeypatch.setattr(PdfExtractor, "extract_metadata", pdf)
@@ -133,7 +136,9 @@ class TestSplitEdition:
             ),
         ],
     )
-    def test_editions_are_split_out(self, title: str, stripped: str, edition: int) -> None:
+    def test_editions_are_split_out(
+        self, title: str, stripped: str, edition: int
+    ) -> None:
         assert split_edition(title) == (stripped, edition)
 
     @pytest.mark.parametrize(
@@ -171,7 +176,10 @@ class TestEditionFromFiles:
         """The PDF fixture calls itself a 2nd edition in its own metadata title."""
         metadata = await Extractor.extract_metadata([PDF])
 
-        assert metadata["title"] == "The Project Gutenberg eBook #33283: Calculus Made Easy"
+        assert (
+            metadata["title"]
+            == "The Project Gutenberg eBook #33283: Calculus Made Easy"
+        )
         assert metadata["edition"] == 2