Annotate structs that may end up in responses

[FoseFx]

Describe the issue

I notice when working on #105 and #106 that it's already impossible to figure out if a struct (e.g. a model) may be used in responses or not. In this case I probably just added more tags that we need to, but it may cause more severe consequences in the future. We should add a comment to all structs that are used in responses (and in which ones).