Add contacts to broadcast

curl --request POST \
  --url https://api.zavu.dev/v1/broadcasts/{broadcastId}/contacts \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "contacts": [
    {
      "recipient": "+14155551234",
      "templateVariables": {
        "name": "John",
        "order_id": "ORD-001"
      }
    },
    {
      "recipient": "+14155555678",
      "templateVariables": {
        "name": "Jane",
        "order_id": "ORD-002"
      }
    }
  ]
}
'

{
  "added": 123,
  "duplicates": 123,
  "invalid": 123,
  "errors": [
    {
      "recipient": "<string>",
      "reason": "<string>"
    }
  ]
}