Use consistent id param type in private.ts

[jpmckinney]

4 times it's id: number, 4 times it's id: string. It'd be better to be consistent, to avoid accidental errors.

When it's number, the caller does e.g. downloadDocumentFn(Number(id)), except for applicationStartFn, which I guess casts as number using useMutation (the others use useQuery).