feat: 增加每日内存回收与 malloc arena 限制

流媒体会话会在 glibc malloc arena 中留下大量不归还操作系统的冷匿名内存，实测单个
服务可累积到 2.5 GB 并占满整个 swap 文件（33 个 64/128 MB 匿名段，空闲时不增长，
随会话单调抬升）。本次从根因与兜底两侧处理：

- systemd/maa-online-server.service: 设置 MALLOC_ARENA_MAX=2，把线程 arena 数量
  从 8xCPU 压到 2，抑制 arena 膨胀
- systemd/maa-daily.service: 单元结束时经 ExecStopPost 重启 API 服务，回收当次会话残留
- systemd/maa-online-recycle.service, maa-online-recycle.timer: 新增每日 03:59（游戏
  每日刷新前）独立回收定时器，错开更新与日常任务窗口，Persistent=false 避免补跑撞车
- bin/maa-online-can-recycle: 回收守卫，检测日常任务（systemd 单元或手工运行）是否
  进行中，是则跳过回收，避免重启打断任务
- bin/maa-online-install-systemd: 安装并启用新增的回收单元
- README.md, user_config/README.md: 补充内存回收机制与定时器说明

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)
+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)
 
 if [ "$(id -u)" != "0" ]; then
     echo "error: must run as root (writes to $DST)" >&2
@@ -25,15 +25,15 @@ done
 
 systemctl daemon-reload
 systemctl enable --now maa-online-server.service
-systemctl enable maa-daily.timer maa-update.timer
-systemctl start maa-daily.timer maa-update.timer
+systemctl enable maa-daily.timer maa-update.timer maa-online-recycle.timer
+systemctl start maa-daily.timer maa-update.timer maa-online-recycle.timer
 
 echo
-systemctl --no-pager list-timers maa-daily.timer maa-update.timer
+systemctl --no-pager list-timers maa-daily.timer maa-update.timer maa-online-recycle.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
+  next runs:   systemctl list-timers maa-daily.timer maa-update.timer maa-online-recycle.timer
 EOF