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_library_audit.py

@@ -9,7 +9,6 @@ from PIL import Image
 from sqlalchemy import select
 
 import backend.app.tasks.library_audit  # noqa: F401 — celery registration
-from backend.app import celery_app
 from backend.app.models import ImageRecord, LibraryAuditRun
 
 pytestmark = pytest.mark.integration
@@ -37,7 +36,7 @@ def _mk_image(db_sync, tmp_path, *, mode, color, name):
 
 
 def test_scan_library_for_rule_populates_matched_ids_for_transparency(
-    db_sync, tmp_path, monkeypatch,
+    db_sync, tmp_path, celery_conf,
 ):
     transparent_rec, _ = _mk_image(
         db_sync, tmp_path, mode="RGBA", color=(0, 0, 0, 0), name="trans.png",
@@ -53,10 +52,10 @@ def test_scan_library_for_rule_populates_matched_ids_for_transparency(
     db_sync.commit()
     audit_id = audit.id
 
-    monkeypatch.setattr(celery_app.celery.conf, "task_always_eager", True)
+    celery_conf(task_always_eager=True)
     from backend.app.tasks.library_audit import scan_library_for_rule
     scan_library_for_rule.run(audit_id)
-    monkeypatch.setattr(celery_app.celery.conf, "task_always_eager", False)
+    celery_conf(task_always_eager=False)
 
     matched = db_sync.execute(
         select(LibraryAuditRun.matched_ids).where(LibraryAuditRun.id == audit_id)
@@ -124,7 +123,7 @@ def test_scan_resumes_and_accumulates_matched(db_sync, tmp_path, monkeypatch):
 
 
 def test_scan_library_for_rule_skips_missing_files_gracefully(
-    db_sync, tmp_path, monkeypatch,
+    db_sync, tmp_path, celery_conf,
 ):
     rec, path = _mk_image(
         db_sync, tmp_path, mode="RGBA", color=(0, 0, 0, 0), name="ghost.png",
@@ -138,10 +137,10 @@ def test_scan_library_for_rule_skips_missing_files_gracefully(
     db_sync.commit()
     audit_id = audit.id
 
-    monkeypatch.setattr(celery_app.celery.conf, "task_always_eager", True)
+    celery_conf(task_always_eager=True)
     from backend.app.tasks.library_audit import scan_library_for_rule
     scan_library_for_rule.run(audit_id)
-    monkeypatch.setattr(celery_app.celery.conf, "task_always_eager", False)
+    celery_conf(task_always_eager=False)
 
     status = db_sync.execute(
         select(LibraryAuditRun.status).where(LibraryAuditRun.id == audit_id)