fix(deploy): copy pruned node_modules from builder to runtime instead of reinstalling

deploy/Dockerfile.backend

@@ -5,11 +5,14 @@ COPY backend/package*.json backend/
 RUN sh -c 'if [ -f backend/package-lock.json ] || [ -f backend/npm-shrinkwrap.json ]; then npm ci --prefix backend --no-audit --no-fund --prefer-offline --no-progress; else npm install --prefix backend --no-audit --no-fund --prefer-offline --no-progress; fi'
 COPY backend/ backend/
 RUN sh -c 'NODE_OPTIONS="--max_old_space_size=512" npm run build --prefix backend'
+## Prune dev dependencies to shrink runtime layer
+RUN npm prune --omit=dev --prefix backend && rm -rf /root/.npm /tmp/*
 
 FROM node:20-alpine
 WORKDIR /app
+ENV NODE_ENV=production
 COPY --from=builder /app/backend/dist ./dist
+COPY --from=builder /app/backend/node_modules ./node_modules
 COPY backend/package*.json ./
-RUN npm ci --production --prefix /app
 EXPOSE 4000
 CMD ["node", "./dist/index.js"]