Migrate an Inngest TypeScript app from SDK v3 to SDK v4

Documented steps

1. Confirm the target: TypeScript SDK v4 reached GA in March 2026; v3 reference pages are now labelled '(Legacy)'. Ref: https://www.inngest.com/changelog/2026-03-17-typescript-sdk-v4-ga
2. Upgrade the dependency (`npm i inngest@latest`) on a branch and run a typecheck first — most v4 breakages surface as type errors.
3. Rewrite every `createFunction` call from the 3-arg form to the 2-arg form: `createFunction({ id }, { event: "x" }, handler)` becomes `createFunction({ id, triggers: { event: "x" } }, handler)`. Multiple triggers become an array under `triggers`.
4. Move `baseUrl`, `fetch`, `signingKey` and `signingKeyFallback` out of `serve()` options and into the `new Inngest({ ... })` constructor.
5. Replace the removed `EventSchemas` class with `eventType()` / `staticSchema()` for typed events.
6. Remove all reads of `event.user` inside handlers — move that data into `event.data` at send time.
7. Update `step.invoke()` call sites: v4 no longer accepts a raw string function reference; pass the function object or a reference helper.
8. Review serverless timeouts: checkpointing is enabled by default in v4 — configure `maxRuntime` where the platform enforces a short execution ceiling.
9. Deploy to a preview/branch environment, resync the app, and replay a representative run before promoting.