Support long-running queries like pageInsights with subscriptions or @stream

[mattkrick]

pageInsights can take a LONG time, longer than 30 seconds.
we have options:

• use the openAI streaming API and stream chunks (LLMs use SSE, we could use that or websockets)
• persist the answer to a DB & have the client re-request it after timeoute
• send intermittent messages saying it's coming soon & then send the final result via subscription

the best option is probably the first because that's what people are used to.
to do that, we could use graphql subscriptions, or the new @stream directive. The latter is the natural choice, because that means we wouldn't have to change our schema based on the whether or not we want the data to come in chunks. Unfortunately, @stream won't be ready until graphql-js v17. So, if we want to implement this now, we should use subscriptions since we've already got that pattern set up. If we can postpone, we should wait for GraphQL v17.

Sources:

• GraphQL v17 (currently in alpha): https://github.com/graphql/graphql-js/releases
• Migration: https://www.graphql-js.org/docs/defer-stream/
• RFC: https://github.com/graphql/graphql-wg/blob/main/rfcs/DeferStream.md#stream
• Relay: https://relay.dev/docs/glossary/#stream
• Spec discussion: https://github.com/graphql/defer-stream-wg/discussions