Add verification step to check pre-built files exist

Dockerfile

@@ -3,7 +3,7 @@ FROM node:20-slim
 # Set noninteractive installation
 ENV DEBIAN_FRONTEND=noninteractive
 
-# Build timestamp to force cache invalidation: 2026-01-12T20:10
+# Build timestamp to force cache invalidation: 2026-01-12T20:20
 
 # Install build dependencies
 RUN apt-get update && apt-get install -y \
@@ -33,6 +33,10 @@ RUN npm install --omit=dev && \
 # - Backend: backend/dist/backend/src/server.js
 # - Parser: public/lib/tgnParser.cjs
 
+# Verify pre-built files exist
+RUN ls -la backend/dist/backend/src/server.js && \
+    ls -la app/dist/index.html
+
 # Set environment variables for Hugging Face Spaces
 ENV PORT=7860
 ENV HOST=0.0.0.0