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/server/handlers.go

@@ -208,6 +208,33 @@ func handleDownloadFile(ctx *gin.Context) {
 	ctx.FileAttachment(path, filename)
 }
 
+func handleDownloadThumbnail(ctx *gin.Context) {
+	boxID := ctx.Param("id")
+	fileID := ctx.Param("file_id")
+	if !boxstore.ValidBoxID(boxID) {
+		ctx.String(http.StatusBadRequest, "Invalid box id")
+		return
+	}
+
+	if _, _, authorized := authorizeBoxRequest(ctx, boxID, true); !authorized {
+		return
+	}
+
+	path, ok := boxstore.ThumbnailFilePath(boxID, fileID)
+	if !ok {
+		ctx.String(http.StatusBadRequest, "Invalid thumbnail")
+		return
+	}
+
+	if _, err := os.Stat(path); err != nil {
+		ctx.String(http.StatusNotFound, "Thumbnail not found")
+		return
+	}
+
+	ctx.Header("Content-Type", "image/jpeg")
+	ctx.File(path)
+}
+
 func handleCreateBox(ctx *gin.Context) {
 	boxID, err := boxstore.NewBoxID()
 	if err != nil {