Unlink Gl Collision Link

Withdraw an invoice-number collision so its document can be booked — accountant.

ZCT-259. The collision row was terminal: it excluded the incoming document from the AI coding queue permanently and manual coding refused it on the document-number pre-check, so a genuinely different supplier invoice could not be booked by any route. Withdrawing the link returns it to both, and the linker will not re-create it.

The payable is not touched — no status change, no ledger write. 409 when the link is already withdrawn, or when it is any tier other than an advisory collision (every other tier records that the document is already in the ledger, so releasing it would book the same liability twice). 404 for an unknown or other-tenant id.