feat: 增加内存观测日志

- bin/maa-online-memlog: 每分钟采集一次服务 cgroup 匿名内存、swap 占用、进程 RSS、
  以及大于 32 MB 的匿名映射数量（glibc arena 膨胀指纹），追加写入 logs/memlog.csv，
  超过 5 MB 自动滚动并保留一代；同时输出一行到 journald
- systemd/maa-online-memlog.service, maa-online-memlog.timer: 每分钟触发采集，
  Nice=10 避免影响流媒体负载
- bin/maa-online-install-systemd: 安装并启用观测定时器，安装提示补充内存日志路径
- README.md, user_config/README.md: 补充内存观测说明、字段表与查看命令
- .gitignore: 忽略运行期产生的 logs/

bin/maa-online-install-systemd

@@ -7,7 +7,7 @@ set -euo pipefail
 ROOT="${MAA_ONLINE_ROOT:-$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)}"
 SRC="$ROOT/systemd"
 DST="${MAA_ONLINE_SYSTEMD_DIR:-/etc/systemd/system}"
-UNITS=(maa-online-server.service maa-daily.service maa-daily.timer maa-update.service maa-update.timer maa-online-recycle.service maa-online-recycle.timer)
+UNITS=(maa-online-server.service maa-daily.service maa-daily.timer maa-update.service maa-update.timer maa-online-recycle.service maa-online-recycle.timer maa-online-memlog.service maa-online-memlog.timer)
 
 if [ "$(id -u)" != "0" ]; then
     echo "error: must run as root (writes to $DST)" >&2
@@ -25,15 +25,16 @@ done
 
 systemctl daemon-reload
 systemctl enable --now maa-online-server.service
-systemctl enable maa-daily.timer maa-update.timer maa-online-recycle.timer
-systemctl start maa-daily.timer maa-update.timer maa-online-recycle.timer
+systemctl enable maa-daily.timer maa-update.timer maa-online-recycle.timer maa-online-memlog.timer
+systemctl start maa-daily.timer maa-update.timer maa-online-recycle.timer maa-online-memlog.timer
 
 echo
-systemctl --no-pager list-timers maa-daily.timer maa-update.timer maa-online-recycle.timer
+systemctl --no-pager list-timers maa-daily.timer maa-update.timer maa-online-recycle.timer maa-online-memlog.timer
 cat <<EOF
 
 MAA Online systemd units are active.
   server log:  journalctl -u maa-online-server -f
   daily log:   journalctl -u maa-daily -n 50
-  next runs:   systemctl list-timers maa-daily.timer maa-update.timer maa-online-recycle.timer
+  memory log:  tail -f $ROOT/logs/memlog.csv
+  next runs:   systemctl list-timers 'maa-*'
 EOF