用途：解锁某一条历史里的付费/私密消息。
请求体：
{
  "messageId": "消息ID"
}
需要带登录 token：
Authorization: Bearer <token>
Content-Type: application/json
返回示例：
{
  "code": 200,
  "message": "success",
  "success": true,
  "data": {
    "unlocked": true,
    "content": "完整消息内容",
    "reason": "ok",
    "creditBalance": 9180,
    "creditsCharged": 10,
    "requiredCredits": 10,
    "shortfallCredits": 0,
    "lockDetail": {
      "locked": false,
      "showContent": true,
      "showUpgrade": false,
      "reason": null,
      "hint": null,
      "detail": null
    }
  }
}
如果积分不够，会返回 unlocked:false，并带 requiredCredits / shortfallCredits / creditBalance。