Bulk upsert contacts

curl --request POST \
  --url http://localhost:8080/api/v1/client/contacts/bulk \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "contacts": [
    {
      "client_contact_id": "crm-001",
      "company": "Acme Corp",
      "custom_fields": {},
      "email": "jane@example.com",
      "external_id": "crm-001",
      "first_name": "Jane",
      "is_subscribed": true,
      "last_name": "Doe",
      "last_purchase_at": "<string>",
      "phone": "+1-555-0100",
      "signup_source": "organic",
      "timezone": "America/New_York",
      "unsubscribe_reason": "<string>",
      "unsubscribed_at": "<string>"
    }
  ],
  "overwrite_blank_fields": true
}
'

{
  "data": {},
  "errors": [
    {
      "condition": "format",
      "error": "Invalid email format",
      "key": "email"
    }
  ],
  "message": "Operation successful",
  "meta": {},
  "success": true
}