Generate a presigned URL for conversation upload

curl --request POST \
  --url https://api.rulebase.co/conversations/upload/presign \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "upload": {
    "filename": "conversation.pdf",
    "source": "outreach_kaia"
  }
}
'

{
  "upload": {
    "id": "<string>",
    "url": "<string>"
  }
}