Adds InBehandeling + Afgewezen statuses and TakeIntoBehandeling(). Generalises the behandelaar decision: Approve() (requires a zaak) and new Reject() both act on an INGEDIEND or IN_BEHANDELING registration, moving it to Ingeschreven/Afgewezen. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>