Add Companies To List

curl -X POST "https://api.clustr-ai.com/api/public/lists/l1a2b3c4-.../companies" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"company_ids": ["9c4d5e6f-...", "7a1b2c3d-..."]}'

{
  "success": true,
  "data": { "message": "Companies added to list", "added": 2 }
}

{
  "success": false,
  "error": { "message": "list not found", "code": "NOT_FOUND" }
}