re-running seed script inserts duplicate invoices

[murfett-au]

Each time npm run seed is executed, an extra 15 rows are inserted into the invoices table.

This is because:

1. no member of the invoices array exported from the placeholder include invoices[n].id; and
2. The id field is not included in the sql insert statement in the seed.ts script.

I have created https://github.com/vercel/next-learn/pull/537 to address these issues.