Add comments_count field to Card API responses

[okdas]

The web UI already shows comment counts on kanban cards (added after discussion #2157), but the API doesn't include this field. The card JSON response only has comments_url, no numeric count.

This means API consumers and CLI tools have to paginate through the full comments endpoint just to know whether a card has any comments at all.

Would be great to add a comments_count integer field to the card response (both list and show), matching what's already visible in the UI.

Any objections?

[flavorjones]

@okdas I think that's a great idea. Are you interested in opening a pull request?