Add a milestone to a project

curl --request POST \
  --url https://slog.ai/projects/{id}/milestones \
  --header 'Content-Type: application/json' \
  --header 'slog-api-key: <api-key>' \
  --data '
{
  "milestoneId": "mile_01jqmile0001"
}
'

{
  "milestone": {
    "id": "mile_01jqmile0001",
    "reference": "ENG-M1",
    "name": "Beta Launch",
    "description": "First public beta release.",
    "startDate": "2026-05-01",
    "dueDate": "2026-07-01",
    "labels": [
      {
        "id": "label_01jqlabel001",
        "name": "bug",
        "color": "#EF4444"
      }
    ],
    "createdAt": "2026-04-01T08:00:00.000Z",
    "updatedAt": "2026-05-01T10:00:00.000Z"
  }
}