🐛(backend) fix dependencies conflicts

Upgrading Django to 5.1 created a severe issue, breaking
mail template-associated features.

The issue originated from the third party 'easy_thumbnail'.
Please refer to the issue #641 on Django's repo. This is
the suggested workaround by @Miketsukami.

src/backend/people/settings.py

@@ -286,6 +286,7 @@ class Base(Configuration):
     # Easy thumbnails
     THUMBNAIL_EXTENSION = "webp"
     THUMBNAIL_TRANSPARENCY_EXTENSION = "webp"
+    THUMBNAIL_DEFAULT_STORAGE_ALIAS = "default"
     THUMBNAIL_ALIASES = {}
 
     # Celery