nit: move response types out of the models package

[lucat1]

While scanning through the code I saw that API response types where defined in models, which should hold the DB structures. I think defining the API types in there is bad practice, we want to have separation between the internal data structures and what's exposed in the API.

This PR does some refactoring and moves all structures defined in models that belong to the API interface in the api package.
I have a feeling (but didn't check) the types in model are also being directly JSON serialized (I get this feeling from the inlining of the gorm.Model struct). This should also be avoided for the same reason as before.
I think that kind of refactoring can probably be done by claude code/gemini without human intervention, but I didn't bother for this PR.

As a side note: What is the purpose of AnswerVersions? why is the name plural? I assume the Answer field is a reference to the Answer ID column. Shouldn't it be marked as such in SQL through gorm annotations?

This is just me raising what are clearly nits. If code quality is not a priority right now, just ignore them and feel free to close this PR :)

[leonardo3130]

Changes on models seem good to me. I initially moved them all inside models.go thinking it was a good thing, but separation between API structs and internal data structures is more important, as you said.

The purpose of AnswerVersions is to keep track of the history of an answer (#27 ).
The name is plural because I made a mistake.
You are also right on the missing gorm annotation.

As I said before, these changes can be merged in my opinion. Let's see what @samuelemusiani has to say.

[samuelemusiani]

Thanks Luca!