Add PVForecastSolcast for the Solcast rooftop-site API. The operator registers a
site in the Solcast web app and enters the API key + resource (site) id:
GET /rooftop_sites/{site_id}/forecasts. pv_estimate (kW) is converted to watts
and fed as pvforecast_ac_power; the timestamp is normalised to the period start
(period_end - period) so it aligns with the resample axis. period_end is UTC.
Completes the set of selectable cloud PV forecast providers
(Akkudoktor, VRM, Import, pvnode, Forecast.Solar, Solcast). Adds tests for the
kW->W conversion, period-start normalisation, ISO-8601 period parsing, the
request URL/auth and HTTP-error handling.
