fix npm bug

backend/Dockerfile

@@ -1,12 +1,17 @@
+# Build context is the repo root (npm workspaces share one root package-lock.json)
 FROM node:20-alpine
 
 ENV NODE_ENV=production
-WORKDIR /backend
+WORKDIR /app
 
-COPY package*.json ./
-RUN npm ci --omit=dev
+COPY package.json package-lock.json ./
+COPY frontend/package.json frontend/
+COPY admin/package.json admin/
+COPY backend/package.json backend/
+RUN npm ci -w backend --omit=dev
 
-COPY . .
+COPY backend/ backend/
 
+WORKDIR /app/backend
 EXPOSE 8004
-CMD ["npm", "start"]
+CMD ["npm", "start"]