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/routers/admin.py

@@ -23,7 +23,7 @@ from services.demo_media import (
     DEMO_DICOM_PROJECT_NAME,
     DEMO_VIDEO_PROJECT_NAME,
     create_parsed_dicom_demo_project,
-    create_unparsed_video_demo_project,
+    create_parsed_video_demo_project,
     demo_dicom_files,
 )
 from services.default_templates import restore_default_templates
@@ -252,13 +252,12 @@ def reset_demo_factory(
 
     restored_templates = restore_default_templates(db)
 
-    video_project = create_unparsed_video_demo_project(
+    video_project = create_parsed_video_demo_project(
         db,
         owner=preserved_admin,
         video_path=settings.demo_video_path,
         project_name=DEMO_VIDEO_PROJECT_NAME,
     )
-    video_project.frame_count = 0
 
     dicom_project = create_parsed_dicom_demo_project(
         db,