async_added_to_hass listed the switchable boxes inline, one fetch per box
popup, so on a slower connection the platform setup ran past
SLOW_SETUP_MAX_WAIT and the whole config entry was cancelled into
setup_error — every entity unavailable despite valid credentials. Move the
listing to a background task so setup never blocks on it; the options fill in
once it returns.
Also bound every portal request to a 30s timeout so one hung request can no
longer drag a refresh, or a first setup, past Home Assistant's own limits and
fail it outright.
Regression covered in tests/test_setup_hygiene.py.
