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_bookshelf_service.py

@@ -2,19 +2,12 @@
 
 import pytest
 
+from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from chitai.services import ShelfService
 from chitai.database import models as m
-
-
-import pytest
-from sqlalchemy import select
-
-from chitai.services.bookshelf import ShelfService
-from chitai.services import BookService
 from chitai.database.models.book_list import BookList, BookListLink
-from chitai.database import models as m
 
 
 @pytest.fixture