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_user_service.py

@@ -21,7 +21,7 @@ class TestUserServiceAuthentication:
 
         # Create a user with a known password
         password = "password123"
-        user = m.User(email=f"test@example.com", password=password)
+        user = m.User(email="test@example.com", password=password)
 
         session.add(user)
         await session.commit()
@@ -52,7 +52,7 @@ class TestUserServiceAuthentication:
 
         # Create user
         password = "password123"
-        user = m.User(email=f"test@example.com", password=password)
+        user = m.User(email="test@example.com", password=password)
 
         session.add(user)
         await session.commit()
@@ -85,7 +85,7 @@ class TestUserServiceCRUD:
     ) -> None:
         """Test getting user by email."""
 
-        user = m.User(email=f"test@example.com", password="password123")
+        user = m.User(email="test@example.com", password="password123")
 
         session.add(user)
         await session.commit()
@@ -102,12 +102,12 @@ class TestUserServiceCRUD:
         """Test creating a new user with a duplicate email."""
 
         # Create first user
-        user = m.User(email=f"test@example.com", password="password123")
+        user = m.User(email="test@example.com", password="password123")
         session.add(user)
         await session.commit()
 
         # Create second user
-        user = m.User(email=f"test@example.com", password="password12345")
+        user = m.User(email="test@example.com", password="password12345")
 
         with pytest.raises(IntegrityError) as exc_info:
             session.add(user)