日志按天分割并自动清理：app-YYYYMMDD.log，保留天数可配

- dailyFileWriter 按天轮转：跨天自动切换到新文件（app-20260804.log）
- LOG_RETAIN_DAYS 控制保留天数（默认 30），到期自动删除
- docker-compose 透传 LOG_RETAIN_DAYS，README 更新环境变量说明
- 补充轮转/清理单测

docker-compose.yml

@@ -47,6 +47,7 @@ services:
       - OPEN_SIGN_SKEW=${OPEN_SIGN_SKEW:-300}
       - OPEN_API_DEBUG=${OPEN_API_DEBUG:-false}
       - LOG_FILE=${LOG_FILE:-logs/app.log}
+      - LOG_RETAIN_DAYS=${LOG_RETAIN_DAYS:-30}
       - DELIVERY_BASE_URL=${DELIVERY_BASE_URL:-}
       - DELIVERY_BFF_BASE_URL=${DELIVERY_BFF_BASE_URL:-https://www.jxya.top/bff}
       - DELIVERY_CHANNEL=${DELIVERY_CHANNEL:-dlc}