API for email notification management (opt in/out)

Add the ability to subscribe and unsubscribe users from post notifications via API.

We notify customers when a feature they've requested moves to a new stage, which is a key driver of portal engagement. However, we also register feature requests on behalf of potential customers during the sales process - before they're officially onboarded. We don't want them receiving notifications at that stage, but we do want to turn them back on once they go live.

Without programmatic control over subscriptions, we have to manage this manually, which doesn't scale with our chatbot-driven workflows. Being able to toggle subscriptions via API would let us automate this entirely.