Add /api/debug/thumbnail/:name endpoint to verify thumbnail file status

This endpoint checks if a thumbnail file exists, its size, and the URL
to access it via the static file route.

index.ts

@@ -945,6 +945,24 @@ const server = Bun.serve({
         });
       },
     },
+    "/api/debug/thumbnail/:name": {
+      GET: async (req) => {
+        const name = req.params.name;
+        const thumbName = `${name}.thumb.png`;
+        const thumbPath = join(process.cwd(), "public/avatars", thumbName);
+        const file = Bun.file(thumbPath);
+        const exists = await file.exists();
+        const size = exists ? file.size : 0;
+        return Response.json({ 
+          name,
+          thumbName,
+          thumbPath,
+          exists,
+          size,
+          url: `/avatars/${thumbName}`
+        });
+      },
+    },
     "/api/avatars": {
       GET: async () => {
         const names = await listAvatars();