修复演示恢复项目帧数据

- 恢复演示出厂设置后直接解析演视LC视频序列并生成可打开帧序列

- 保持演视DICOM序列按文件名自然顺序恢复并生成帧

- 增加 MinIO 浏览器访问端点配置，修复 Docker 部署中封面和帧图预签名地址使用容器内主机名的问题

- 更新管理员恢复测试覆盖视频和 DICOM 帧数量

- 更新 README 和前后端契约/设计/测试文档中的演示恢复说明

backend/minio_client.py

@@ -14,6 +14,7 @@ logger = logging.getLogger(__name__)
 BUCKET_NAME = "seg-media"
 
 _minio_client: Optional[Minio] = None
+_minio_public_client: Optional[Minio] = None
 
 
 def get_minio_client() -> Minio:
@@ -29,6 +30,20 @@ def get_minio_client() -> Minio:
     return _minio_client
 
 
+def get_minio_public_client() -> Minio:
+    """Return a MinIO client configured for browser-facing presigned URLs."""
+    global _minio_public_client
+    if _minio_public_client is None:
+        endpoint = settings.minio_public_endpoint or settings.minio_endpoint
+        _minio_public_client = Minio(
+            endpoint,
+            access_key=settings.minio_access_key,
+            secret_key=settings.minio_secret_key,
+            secure=settings.minio_secure,
+        )
+    return _minio_public_client
+
+
 def ensure_bucket_exists() -> None:
     """Create the bucket if it does not already exist."""
     client = get_minio_client()
@@ -97,7 +112,7 @@ def get_presigned_url(
     Returns:
         Presigned URL string.
     """
-    client = get_minio_client()
+    client = get_minio_public_client()
     try:
         url = client.get_presigned_url(method, BUCKET_NAME, object_name, expires=timedelta(seconds=expires))
         return url