feat: 视频缩略图生成、最近文件面板与前端视图重构

- 新增 VideoThumbnailService，基于 ffmpeg 截取视频缩略图，ffprobe 提取时长
  - 新增 ManagedThumbnailMap 模型及多数据库迁移，存储缩略图元数据
  - 新增 /api/thumbnails/{id} 缩略图流端点
  - 新增最近添加/最近播放 API 与前端面板，支持列表/网格双视图切换
  - FileRecordDto 扩展 thumbnailUrl、videoDuration、lastPlayedAt 字段
  - 前端新增文件库 Tab 导航、卡片网格视图、视频海报与时长信息栏
  - 添加文件库目录不再同步全量扫描，改为后台异步自动扫描

FileShare-Services/Endpoints/AppEndpoints.cs

@@ -69,6 +69,14 @@ namespace FileShare_Services.Endpoints
                     .WithOpenApi("FileLibrary", "浏览文件库目录结构。")
                     .WithName("BrowseDirectory");
 
+                endpoints.MapGet<IFileLibraryEndpointService, RecentFilesRequest>("api/files/recent", (service, request, _) => service.GetRecentFilesAsync(request))
+                    .WithOpenApi("FileLibrary", "获取最近添加或最近播放的文件。")
+                    .WithName("GetRecentFiles");
+
+                endpoints.MapPost<IFileLibraryEndpointService, MarkFilePlayedRequest>("api/files/played", (service, request, _) => service.MarkFilePlayedAsync(request))
+                    .WithOpenApi("FileLibrary", "标记文件已播放。")
+                    .WithName("MarkFilePlayed");
+
                 endpoints.MapGet<IFileLibraryEndpointService, FileQueryRequest>("api/files/detail", (service, request, _) => service.GetFileAsync(request))
                     .WithOpenApi("FileLibrary", "查询文件详情。")
                     .WithName("GetFileDetail");