Fix: keep Celery's Django task base for file move scoped tasks

Passing task_cls to Celery replaces the Django fixup's default DjangoTask, so FileMoveScopedTask, built on the plain Task, left every task without delay_on_commit. The workflow AI suggestions trigger failed with an AttributeError.

FileMoveScopedTask now subclasses DjangoTask, which only needs django.db.transaction and is safe to import before apps load. The task scope test also asserts tasks are DjangoTask instances.

src/paperless/celery.py

@@ -1,7 +1,7 @@
 import os
 
 from celery import Celery
-from celery import Task
+from celery.contrib.django.task import DjangoTask
 from celery.signals import worker_process_init
 from kombu.serialization import register
 
@@ -28,12 +28,15 @@ register(
 )
 
 
-class FileMoveScopedTask(Task):
+class FileMoveScopedTask(DjangoTask):
     """
     Run every task inside a document file move scope, so each document a task
     changes is moved once when the task ends, after its writes have committed.
     The scope also flushes when the task raises, for writes that committed
     before the failure.
+
+    Subclasses DjangoTask because a custom task_cls replaces the Django fixup's
+    default, and delay_on_commit lives there.
     """
 
     def __call__(self, *args, **kwargs):