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

static/css/box.css

@@ -124,6 +124,14 @@
     image-rendering: pixelated;
 }
 
+.box-file.has-thumbnail .box-file-icon {
+    width: 40px;
+    height: 32px;
+    object-fit: cover;
+    background: #ffffff;
+    border: 1px solid #808080;
+}
+
 .box-file-name,
 .box-file-meta {
     width: 100%;