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/conftest.py

@@ -2,6 +2,8 @@ from pathlib import Path
 from uuid import uuid4
 import pytest
 
+from chitai import services
+
 from advanced_alchemy.base import UUIDAuditBase
 from litestar.testing import AsyncTestClient
 from sqlalchemy import text
@@ -155,7 +157,6 @@ async def other_authenticated_client(
 
 
 # Service fixtures
-from chitai import services
 
 
 @pytest.fixture