feat: add thumbnail metadata and download endpoint

- Extend `BoxFile` with thumbnail path/status fields and internal URL
- Populate `ThumbnailURL` when a thumbnail path is present during decoration
- Add `/box/:id/thumbnails/:file_id` route and handler to serve JPEG thumbnails
- Introduce thumbnail status constants to standardize processing state reportingfeat: add thumbnail metadata and download endpoint

- Extend `BoxFile` with thumbnail path/status fields and internal URL
- Populate `ThumbnailURL` when a thumbnail path is present during decoration
- Add `/box/:id/thumbnails/:file_id` route and handler to serve JPEG thumbnails
- Introduce thumbnail status constants to standardize processing state reporting

lib/routing/routes.go

@@ -10,6 +10,7 @@ type Handlers struct {
 	BoxStatus          gin.HandlerFunc
 	DownloadBox        gin.HandlerFunc
 	DownloadFile       gin.HandlerFunc
+	DownloadThumbnail  gin.HandlerFunc
 	CreateBox          gin.HandlerFunc
 	ManifestFileUpload gin.HandlerFunc
 	FileStatusUpdate   gin.HandlerFunc
@@ -25,6 +26,7 @@ func Register(router *gin.Engine, handlers Handlers) {
 	router.GET("/box/:id/status", handlers.BoxStatus)
 	router.GET("/box/:id/download", handlers.DownloadBox)
 	router.GET("/box/:id/files/:filename", handlers.DownloadFile)
+	router.GET("/box/:id/thumbnails/:file_id", handlers.DownloadThumbnail)
 
 	router.POST("/box", handlers.CreateBox)
 	router.POST("/box/:id/login", handlers.BoxLoginPost)