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/js/box.js

@@ -16,12 +16,14 @@ function updateBoxFile(file) {
     }
 
     const meta = item.querySelector(".box-file-meta");
+    const icon = item.querySelector(".box-file-icon");
     const isComplete = file.status === "complete";
     const isFailed = file.status === "failed";
 
     item.classList.toggle("is-complete", isComplete);
     item.classList.toggle("is-failed", isFailed);
     item.classList.toggle("is-loading", !isComplete && !isFailed);
+    item.classList.toggle("has-thumbnail", Boolean(file.thumbnail_path));
     item.dataset.status = file.status;
     item.title = file.title;
 
@@ -38,6 +40,10 @@ function updateBoxFile(file) {
     if (meta) {
         meta.textContent = `${file.status_label} · ${file.size_label}`;
     }
+
+    if (icon) {
+        icon.src = file.thumbnail_path || file.icon_path;
+    }
 }
 
 async function refreshBoxStatus() {
@@ -59,7 +65,11 @@ async function refreshBoxStatus() {
         boxStatus.textContent = `${completeCount}/${result.files.length} ready`;
     }
 
-    return result.files.some((file) => file.status === "pending" || file.status === "uploading");
+    return result.files.some((file) => {
+        const isUploading = file.status === "pending" || file.status === "uploading";
+        const isWaitingForThumbnail = file.status === "complete" && !file.thumbnail_status && !file.thumbnail_path;
+        return isUploading || isWaitingForThumbnail || file.thumbnail_status === "processing";
+    });
 }
 
 if (boxPanel) {