API v2: create, fill and authorise standalone Sale Credit Notes

The UI can create a standalone Sale Credit Note (Sales → New → Sale Credit Note) that is not linked to an invoice. API v2 cannot do this:
- POST /sale/creditnote always requires CreditNoteInvoiceNumber and an AUTHORISED invoice.
- A standalone credit note saved as a draft in the UI cannot be filled via the API either ("Invoice with number … not found").
- PUT /sale with a nested CreditNotes array returns HTTP 200 but ignores the content.

Request: let POST /sale/creditnote (or a new endpoint) create a standalone credit note with lines, date, memo and status DRAFT/AUTHORISED, same as the UI form. Credit note lines should accept any service product, not only invoiced SKUs.

Use case: we issue around 100 wholesale credit notes a year (returns, promotions, rebates, price adjustments) that are not tied to a single invoice. Today every one has to be keyed in manually.