🐛(backend) compatibility issue with django and easy_thumbnails

There is a compatibility issue between django 5.1
and easy_thumbnails 2.9.
This commit fixes the issue.

src/backend/impress/settings.py

@@ -314,6 +314,7 @@ class Base(Configuration):
     # Easy thumbnails
     THUMBNAIL_EXTENSION = "webp"
     THUMBNAIL_TRANSPARENCY_EXTENSION = "webp"
+    THUMBNAIL_DEFAULT_STORAGE_ALIAS = "default"
     THUMBNAIL_ALIASES = {}
 
     # Celery