perf,feat(events),ops: third optimization pass

Overview cold start:
- InstanceResourceService persists every live fetch into status_snapshots
  (category 'resources') and serves a snapshot younger than 2 minutes
  before calling upstream, so the first fleet overview after a restart
  costs zero device requests; instance deletion cleans up via FK cascade

Bounded retention for the four fastest-growing tables (15-minute sweep):
- connection_logs: one row per probe per beat had no automatic cleanup
- audit_events: new pruneBefore (90d)
- operation_preparations: every prepare/retry attempt inserted a row,
  terminal rows now expire after 7 days
- notification_queue: terminal rows pruned after 30 days

SSE events now cover instance lifecycle:
- create/update emit instance envelopes from the routes; the two-phase
  delete emits a job envelope on every terminal transition plus an
  instance envelope when the node actually disappears, so fleet and
  instance views invalidate in real time

Linux ops:
- 'install.sh unit' writes systemd user units for API and gateway with
  Restart=on-failure, 0600 secret injection, and
  MULTI_SIMADMIN_SYSTEMD_UNIT wired so console self-update restarts via
  systemctl -- closing the self-update loop on Linux

README.md

@@ -45,6 +45,16 @@ sh /tmp/multi-simadmin-install.sh uninstall
 
 `uninstall` 默认只移除程序源码，保留数据库、Gateway token 和日志。默认安装位置 macOS 为 `~/Library/Application Support/multi-simadmin`，Linux 为 `~/.local/share/multi-simadmin`（遵循 XDG），均可通过 `MULTI_SIMADMIN_HOME` 覆盖；额外 LAN Host 可通过 `MULTI_SIMADMIN_ALLOWED_HOSTS`（逗号分隔）配置。
 
+#### Linux systemd（可选，但推荐）
+
+`sh install.sh unit` 会生成 `~/.config/systemd/user/` 下的 API 与 Gateway 用户单元（`Restart=on-failure`、开机自启、密钥经 0600 的 `service-env` 注入）。随后：
+
+```bash
+systemctl --user enable --now multi-simadmin-api.service multi-simadmin-gateway.service
+```
+
+单元内声明了 `MULTI_SIMADMIN_SYSTEMD_UNIT`，因此 Web 控制台的“在线更新”在 Linux 上会把重启交给 systemd 完成，形成自更新闭环。由 systemd 托管后请用 `systemctl` 管理进程，不要再用本脚本 `start/stop`，避免双进程争抢端口。
+
 ### Fleet 节点总览与跨域健康
 
 `/fleet` 读取本机 Control Plane 中的节点状态、资源快照与设备号码，形成节点总览。侧栏在实例总数 / 在线 / 需处理之外，还聚合跨域健康指标：