fix(deploy): copy package.json into subdirs and run npm with --prefix to handle monorepo-like layout

deploy/Dockerfile.backend

@@ -1,9 +1,9 @@
 FROM node:20-alpine AS builder
 WORKDIR /app
-COPY backend/package*.json ./
-# Use npm ci if a lockfile exists, otherwise fall back to npm install
-RUN sh -c 'if [ -f package-lock.json ] || [ -f npm-shrinkwrap.json ]; then npm ci --production; else npm install --production; fi'
-COPY backend/ ./
+COPY backend/package*.json backend/
+# Use npm ci if a lockfile exists in backend, otherwise fall back to npm install
+RUN sh -c 'if [ -f backend/package-lock.json ] || [ -f backend/npm-shrinkwrap.json ]; then npm ci --prefix backend --production; else npm install --prefix backend --production; fi'
+COPY backend/ backend/
 RUN npm run build --prefix backend
 
 FROM node:20-alpine