fix(backup-service): 修复 builder stage 使用 Alpine 导致 Prisma 失败

问题：
- Builder stage 使用 node:20-alpine
- Prisma 在 Alpine 上下载二进制文件时网络错误
- linux-musl-openssl 兼容性问题

解决：
- Builder 和 Production 都使用 node:20-slim
- 统一使用 Debian base image
- 在 builder stage 安装 OpenSSL

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/services/backup-service/Dockerfile

@@ -1,8 +1,13 @@
 # Stage 1: Build
-FROM node:20-alpine AS builder
+FROM node:20-slim AS builder
 
 WORKDIR /app
 
+# Install OpenSSL for Prisma
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    openssl \
+    && rm -rf /var/lib/apt/lists/*
+
 # Copy package files
 COPY package*.json ./
 COPY prisma ./prisma/