fix: add root Dockerfile for HF Spaces

Dockerfile

@@ -0,0 +1,21 @@
+FROM node:20
+
+WORKDIR /app
+
+# Copy server package.json first to cache layers
+COPY server/package*.json ./
+
+# Install only production dependencies
+RUN npm ci --omit=dev
+
+# Copy all server application files
+COPY server/ .
+
+# Expose port 3000
+EXPOSE 3000
+
+# Set environment to production
+ENV NODE_ENV=production
+
+# Run start command
+CMD ["node", "server.js"]