Refactor user ID handling to use UUIDs; update related components and serializers for consistency

backend/server/main/utils.py

@@ -0,0 +1,10 @@
+from rest_framework import serializers
+
+def get_user_uuid(user):
+    return str(user.uuid)
+
+class CustomModelSerializer(serializers.ModelSerializer):
+    def to_representation(self, instance):
+        representation = super().to_representation(instance)
+        representation['user_id'] = get_user_uuid(instance.user_id)
+        return representation