Fix serialisation of callbacks

Because the IDs of our callback and inbound SMS APIs were stored in
lists instead of directly on the serialised model they weren’t getting
cast to a string before trying to JSONify them. And JSON doesn’t know
what to do with a UUID object.

For some reason this was only affecting the endpoint for fetching
inbound SMS.

app/schemas.py

@@ -63,9 +63,16 @@ class UUIDsAsStringsMixin:
     @post_dump()
     def __post_dump(self, data):
         for key, value in data.items():
+
             if isinstance(value, UUID):
                 data[key] = str(value)
 
+            if isinstance(value, list):
+                data[key] = [
+                    (str(item) if isinstance(item, UUID) else item)
+                    for item in value
+                ]
+
 
 class BaseSchema(ma.ModelSchema):