The discovery beacon previously re-announced at a random 3-10 min interval.
Replace that with a fixed, user-configurable cadence:
- OtaManager::advert_mins() — re-advertise every N minutes after the boot
burst; 0 disables periodic re-advertise (boot burst only). Default 24h.
- Persisted in NodePrefs (CommonCLI) and runtime-tunable: `ota config advert
<minutes>` (0..10080; 0 = off), and shown in `ota config`.
- When periodic advert is disabled, the scheduler still re-checks the config
on a slow timer, so a later `ota config advert <mins>` takes effect live.
Also advertise immediately whenever the served set changes — when a motatool
folder is attached to / detached from the ESP32 WiFi seeder — so peers learn
about newly-available firmware without waiting for the next interval (the
`ota folder` serial path already announced on attach).
Docs: protocol beacon-cadence note + user-guide `ota config advert`.