Add card assignees in card JSON builder

[petzbloom-sudo]

This change adds the "assignees" to the card.json.jbuilder partial.

Currently, the card JSON payload (used by both the API and Webhooks) includes the creator but omits assignment data. This makes it hard for webhook integrations to determine who is responsible for a card.

I have added the "assignees" collection to the standard card JSON response, reusing the existing "users/user" partial to ensure data consistency.

Note to Maintainers I am a "low-code" contributor and do not have a local Rails environment set up to test this. I tried to update the webhook builder Json but it got closed so I'm trying again this way directly in the card.

Thanks, Bloom