When specifying `CronItemOptions`, `backfillPeriod` is not optional in TypeScript

### Summary

I'm defining my cronList as an array of `CronItem`s, so that I can later pass them through `parseCronItems`. This has worked great! (Thank you very much for the library!)

Today, I wanted to add `maxAttempts: 1` to one of my `CronItem`s, and created an `options` object for that. TypeScript started complaining that the options must have a `backfillPeriod` key.

### Steps to reproduce

Use this code:

```ts
import { type CronItem } from 'graphile-worker'

const cronList: CronItem[] = [
{
task: 'example',
match: '* * * * *,
identifier: 'example',
options: {
maxAttempts: 1,
// TypeScript: Property 'backfillPeriod' is missing in type '{ maxAttempts: number; }'
// but required in type 'CronItemOptions'. ts(2741)
},
},
}
```

### Expected results

I'd expect the key to be optional, like the other options in `CronItemOptions`.

### Actual results

At runtime, this seems to work correctly and I don't see any issues. The problem is only the type error.

### Additional context

- Node.js 20.17.0
- TypeScript v5.6.2

### Possible Solution

I think the problem is that Graphile Worker uses the same interface internally and externally. If I understand correctly, the `backfillPeriod` is defaulted to 0 here:

https://github.com/graphile/worker/blob/436e29968d28c48d5f46a012b9c83b0747e2eff1/src/crontab.ts#L151-L153

So it makes sense that _internally_, `backfillPeriod` is always defined. But since this runtime default exists, I think there should be a slightly different interface to the outside.