部署：添加部署配置（nginx、systemd、Dockerfile），并改进会话列表与头像显示

deploy/Dockerfile.backend

@@ -0,0 +1,14 @@
+FROM node:20-alpine AS builder
+WORKDIR /app
+COPY backend/package*.json ./
+RUN npm ci --production
+COPY backend/ ./
+RUN npm run build --prefix backend
+
+FROM node:20-alpine
+WORKDIR /app
+COPY --from=builder /app/backend/dist ./dist
+COPY backend/package*.json ./
+RUN npm ci --production --prefix /app
+EXPOSE 4000
+CMD ["node", "./dist/index.js"]