Structure durable work into Inngest steps with step.run so retries do not repeat completed work

Documented steps

1. Wrap every non-deterministic side effect — database writes, HTTP calls, payments, file writes — in `await step.run("unique-step-id", async () => { ... })`.
2. Return only JSON-serializable data from a step; the return value is persisted into run state and replayed on subsequent attempts.
3. Keep pure, cheap, deterministic code (branching, mapping, formatting) outside steps — it re-executes on every attempt by design and that is fine.
4. Give each step a stable, descriptive id. Inngest memoizes state by step id, so ids must be unique within a run's logical path.
5. For loops, reuse the same step id: the SDK records a per-id counter that increments on each call, so `for (const x of xs) await step.run("process-item", ...)` works without dynamic ids.
6. Treat each `step.run` as a transaction: if the callback throws, the whole step retries from the top, so make step bodies idempotent or single-effect.
7. Verify durability by forcing a mid-function failure locally and confirming in the Dev Server timeline that earlier steps show as memoized rather than re-executed.