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

systemd/maa-online-recycle.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=Recycle the MAA Online cloud game API server (reclaim swapped anonymous memory)
+
+[Service]
+Type=oneshot
+# Never recycle while a daily run is in progress: the timer fires next to the
+# daily window, so a run can already be underway.
+ExecCondition=__ROOT__/bin/maa-online-can-recycle
+# Streaming sessions churn large temporaries across the asyncio/aiohttp/aiortc
+# thread pool; glibc keeps the freed arenas, so a restart is what actually
+# returns the memory (including anything parked in swap) to the OS.
+ExecStart=/bin/systemctl restart maa-online-server.service

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

systemd/maa-online-server.service

@@ -11,6 +11,12 @@ RestartSec=5
 # Belt and braces: never let a proxy environment leak into the cloud gaming
 # server (bin/maa-online-server already unsets these).
 Environment=NO_PROXY=localhost,127.0.0.1,::1
+# Streaming sessions churn large temporaries (video frames, PNG buffers) across
+# the asyncio/aiohttp/aiortc thread pool. glibc gives each thread its own 64 MB
+# malloc arena and never returns it to the OS, so the service used to ratchet up
+# to gigabytes of cold anonymous memory and eventually fill the swap file.
+# Capping the arena count keeps the footprint flat.
+Environment=MALLOC_ARENA_MAX=2
 
 [Install]
 WantedBy=multi-user.target