Fix Django 6.0 compatibility issues

- Set TASKS to django_tasks ImmediateBackend so modelsearch keeps working
  (Django 6.0 ships its own django.tasks.* with a different Task signature
  that lacks `enqueue_on_commit`, which broke the modelsearch @task decorator).
- Bump django-autocomplete-light from 3.9.4 to 4.0.0 (3.9.4 imports the
  removed `django.utils.itercompat` module).

Author: claude

backend/pycon/settings/base.py

@@ -379,6 +379,12 @@
     }
 }
 
+TASKS = {
+    "default": {
+        "BACKEND": "django_tasks.backends.immediate.ImmediateBackend",
+    }
+}
+
 WAGTAIL_SITE_NAME = "cms"
 WAGTAIL_I18N_ENABLED = True
 

backend/pyproject.toml

@@ -64,7 +64,7 @@ dependencies = [
     "countries<1.0.0,>=0.2.0",
     "django-ses==4.1.1",
     "hashids<2.0.0,>=1.2.0",
-    "django-autocomplete-light==3.9.4",
+    "django-autocomplete-light==4.0.0",
     "django-queryinspect<2.0.0,>=1.1.0",
     "lxml==6.1.0",
     "unidecode<2.0.0,>=1.1.1",