docs(operations): felis-api 长时间停摆时日志与告警的实测时间线

  `/opt/felis/velocity/plugins/felis-link/last-servers.json`, the last server list the

  API answered with, until a refresh succeeds (every 15 s).

A longer outage reads like this in the logs [VM-VERIFIED]. The drill scaled felis-api

to 0 for about 8 minutes on the reference VM.

- The proxy logged `server list refresh failed ... keeping current registrations` 11 s

  in, then `still failing: 22 failed attempts over 304 s` at the 5-minute mark.

- The watchdog found `deployment/felis-api` critical on its first run after the scale.

  It raised the alert on the first run past 5 minutes, at about 7 minutes; with no

  `[smtp]` that is logged only (`journalctl -u felis-watchdog`).

- The proxy logged `server list refresh recovered after 32 failed attempts over 469 s`

  as soon as the new pod was Available.

## 2. Sizing

### What the platform itself uses