test: set Celery config through a celery_conf fixture, not monkeypatch

pytest 9.2.0 (released 2026-10-27, changelog #10644) changed how
monkeypatch.setattr restores an attribute the object does not hold in
its own __dict__: undo now deletes it instead of assigning the old value
back. Celery's conf serves every key through __getattr__, so undo called
delattr, the delete raised AttributeError, and pytest skipped the rest
of that test's undos. That made 53 teardown errors across six files
(cleanup API, task-run signals, tasks admin, backup, library audit, the
soft-limit salvage test), and could leak patches into later tests.

CI installs pytest unpinned, so the first run after the release picked it
up: the weekly base refresh of main, 2026-11-01, and every run since. The
gate held, so nothing was published.

tests/conftest.py gains `celery_conf(**values)`: it records each key's
value through conf[key], writes with conf.update(), and restores the
saved values after the test. The six files use it.

Task #6864.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_tasks_backup.py

@@ -10,16 +10,14 @@ from pathlib import Path
 import pytest
 from sqlalchemy import select
 
-from backend.app.celery_app import celery
 from backend.app.models import BackupRun, ImportSettings
 
 pytestmark = pytest.mark.integration
 
 
 @pytest.fixture(autouse=True)
-def _eager_celery(monkeypatch):
-    monkeypatch.setattr(celery.conf, "task_always_eager", True)
-    monkeypatch.setattr(celery.conf, "task_eager_propagates", False)
+def _eager_celery(celery_conf):
+    celery_conf(task_always_eager=True, task_eager_propagates=False)
 
 
 @pytest.fixture(autouse=True)