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_services/test_library_service.py

@@ -34,8 +34,8 @@ class TestLibraryServiceCRUD:
         assert library.name == "Test Library"
         assert library.root_path == library_path
         assert library.path_template == "{author}/{title}"
-        assert library.description == None
-        assert library.read_only == False
+        assert library.description is None
+        assert library.read_only is False
 
         # Check if directory was created
         assert Path(library.root_path).is_dir()
@@ -56,8 +56,8 @@ class TestLibraryServiceCRUD:
             read_only=False,
         )
 
-        with pytest.raises(PermissionError) as exc_info:
-            library = await library_service.create(library_data)
+        with pytest.raises(PermissionError):
+            await library_service.create(library_data)
 
         # Check if directory was created
         assert not Path(library_path).exists()
@@ -86,8 +86,8 @@ class TestLibraryServiceCRUD:
         assert library.name == "Test Library"
         assert library.root_path == library_path
         assert library.path_template == "{author}/{title}"
-        assert library.description == None
-        assert library.read_only == True
+        assert library.description is None
+        assert library.read_only is True
 
     async def test_create_library_read_only_nonexistent_path(
         self, library_service: LibraryService, tmp_path: Path
@@ -138,7 +138,7 @@ class TestLibraryServiceCRUD:
         assert library.root_path == "./books"
         assert library.path_template == "{author}/{title}"
         assert library.description is None
-        assert library.read_only == False
+        assert library.read_only is False
 
     # async def test_delete_library_keep_files(
     #     self, session: AsyncSession, library_service: LibraryService