fix(backup-service): 使用 printf 替代 echo 创建启动脚本

Alpine 的 sh 不支持 echo 的 \n 转义，改用 printf。

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

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

backend/services/backup-service/Dockerfile

@@ -40,12 +40,7 @@ COPY --from=builder /app/node_modules/.prisma ./node_modules/.prisma
 COPY --from=builder /app/prisma ./prisma
 
 # Create startup script that runs migrations before starting the app
-RUN echo '#!/bin/sh\n\
-set -e\n\
-echo "Running database migrations..."\n\
-npx prisma migrate deploy || npx prisma db push --accept-data-loss\n\
-echo "Starting application..."\n\
-exec node dist/src/main.js\n' > /app/start.sh && chmod +x /app/start.sh
+RUN printf '#!/bin/sh\nset -e\necho "Running database migrations..."\nnpx prisma migrate deploy || npx prisma db push --accept-data-loss\necho "Starting application..."\nexec node dist/src/main.js\n' > /app/start.sh && chmod +x /app/start.sh
 
 # Set ownership
 RUN chown -R nestjs:nodejs /app