Per how it's actually run: drop OTA and the fallback AP/captive_portal — this is
a fixed, low-value proxy, so updates and any WiFi recovery are done by re-flashing
over USB. Secrets shrink to iot_wifi_ssid / iot_wifi_password /
shelly_ble_proxy_api_key (the api encryption key stays; it's the ble-agent
handoff). README secrets list + flashing note updated to match.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>