Update Bulk Artist Splits

curl --request POST \
  --url https://server26-dot-royalti-project.uc.r.appspot.com/artist/bulksplit \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "updates": [
    {
      "artistId": "<string>",
      "split": [
        {
          "user": "<string>",
          "share": 123
        }
      ]
    }
  ]
}'

{
  "message": "Bulk split update completed",
  "updatedCount": 5
}