feat: expose closed boolean in card JSON API

[seuros]

Adds closed field to card JSON response, allowing API consumers to detect closed status without parsing the status enum or making additional API calls.

I was working on adding Fizzy Support to ActiveProject gem, and noticed that i need to make an extra call or parse status strings to determine if a card is simply closed or not. Other adapters provide the boolean.

Adding json.closed card.closed? gives a direct boolean, no string comparison needed on the client side.