参与生命周期

已接受 → 进行中 → 已提交 → 已批准
                            → 已拒绝
       → 已超时 (24h)
       → 已退出
→ 已拒绝匹配

{
  "status": "accepted",
  "nextActions": [
    {"action": "complete", "endpoint": "POST /studies/{id}/complete"},
    {"action": "withdraw", "endpoint": "POST /studies/{id}/withdraw"}
  ],
  "flow": "[accepted] -> in_progress -> submitted -> approved"
}