Reject a consolidation proposal

curl --request POST \
  --url https://3ngram-server-production.up.railway.app/api/v1/proposals/{id}/reject \
  --header 'Content-Type: application/json' \
  --header 'X-API-Key: <api-key>' \
  --data '
{
  "rationale": "<string>"
}
'

{
  "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a"
}