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/boxstore/store.go

@@ -329,6 +329,9 @@ func DecorateFile(boxID string, file models.BoxFile) models.BoxFile {
 	}
 
 	file.IconPath = IconForMimeType(file.MimeType, file.Name)
+	if file.ThumbnailPath != nil {
+		file.ThumbnailURL = *file.ThumbnailPath
+	}
 	file.DownloadPath = "/box/" + boxID + "/files/" + url.PathEscape(file.Name)
 	file.UploadPath = "/box/" + boxID + "/files/" + url.PathEscape(file.ID) + "/upload"
 	file.IsComplete = file.Status == models.FileStatusReady