Fix: Downgrade to Express v4 and rebuild deps in Docker

Dockerfile

@@ -15,9 +15,9 @@ RUN npm run build
 FROM node:18-slim
 WORKDIR /app
 
-# Copy backend package config and install production dependencies
-COPY backend/package.json backend/package-lock.json* ./
-RUN npm ci --omit=dev
+# Copy package.json ONLY and install production dependencies from scratch
+COPY backend/package.json ./
+RUN npm install --omit=dev
 
 # Copy rest of the backend code
 COPY backend/ .