feat: add encrypted_dek field to Letter model and enforce its presence during serialization

backend/letters/serializers.py

@@ -12,6 +12,7 @@ class LetterSerializer(serializers.ModelSerializer):
             "status",
             "encrypted_content",
             "encrypted_metadata",
+            "encrypted_dek",
             "unlock_at",
             "sealed_at",
             "created_at",
@@ -22,3 +23,10 @@ class LetterSerializer(serializers.ModelSerializer):
     def create(self, validated_data):
         user = self.context["request"].user  # get user from access token
         return Letter.objects.create(user=user, **validated_data)
+
+    def validate(self, data):
+        if (data.get("encrypted_content") or data.get("encrypted_metadata")) and not data.get("encrypted_dek"):
+            raise serializers.ValidationError(
+                "encrypted_dek is required when encrypted_content and encrypted_metadata are present"
+            )
+        return data