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-daily.service

@@ -9,6 +9,10 @@ ExecStart=__ROOT__/bin/maa-online-daily-run
 # A normal run takes about 25 minutes; the runner's own per-attempt watchdog
 # aborts a hung session well before this ceiling.
 TimeoutStartSec=7200
-# Safety net: whatever stops the unit (success, failure, timeout), make sure
-# the remote cloud session is released.
+# Safety net: whatever stops the unit (success, failure, timeout), release the
+# remote cloud session first...
 ExecStopPost=/bin/sh -c "/usr/bin/curl --noproxy '*' -fsS -X POST http://127.0.0.1:22888/exit || true"
+# ...then recycle the API server. A streaming session leaves gigabytes of cold
+# anonymous memory in glibc malloc arenas that is never returned to the OS;
+# without a recycle it accumulates until the whole swap file is used up.
+ExecStopPost=/bin/systemctl --no-block restart maa-online-server.service