fix: complaint API create — due_at no longer required (auto-calculated by model.save())

The ComplaintSerializer inherited 'required' for due_at from the model field
(not nullable at DB level), but the model's save() method auto-calculates
it via calculate_sla_due_date(). Added extra_kwargs = {'due_at': {'required': False}}
so API callers can omit it. Verified: POST without due_at returns 201 + reference.

apps/complaints/serializers.py

@@ -263,6 +263,9 @@ class ComplaintSerializer(serializers.ModelSerializer):
             "created_at",
             "updated_at",
         ]
+        extra_kwargs = {
+            "due_at": {"required": False},
+        }
 
     def get_domain_details(self, obj):
         """Get domain details"""