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

systemd/maa-online-recycle.timer

@@ -0,0 +1,16 @@
+[Unit]
+Description=Daily recycle of the MAA Online API server just before the Arknights daily reset
+
+[Timer]
+# 03:59 sits immediately ahead of the 04:00 in-game daily reset and of the
+# maa-daily.timer window (04:00 + up to 2h), so the API server is freshly
+# recycled exactly when the daily run starts. Adjust both if the game server's
+# reset time differs.
+OnCalendar=*-*-* 03:59:00
+RandomizedDelaySec=0
+# No catch-up run: a late recycle could otherwise land inside the daily window.
+Persistent=false
+Unit=maa-online-recycle.service
+
+[Install]
+WantedBy=timers.target