The gateway resolves the zaaktype's eindstatus from the catalogus (isEindstatus, falling back to the highest volgnummer) and POSTs a status against the zaak. Exposed as POST /statussen for the domain's approve use case. Adds an integration test that sets the eindstatus against a real OpenZaak and verifies the zaak's current status. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>