feat(prediction): add Solcast PV forecast provider

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.

src/akkudoktoreos/prediction/pvforecast.py

@@ -12,6 +12,7 @@ from akkudoktoreos.prediction.pvforecastforecastsolar import (
 )
 from akkudoktoreos.prediction.pvforecastimport import PVForecastImportCommonSettings
 from akkudoktoreos.prediction.pvforecastpvnode import PVForecastPVNodeCommonSettings
+from akkudoktoreos.prediction.pvforecastsolcast import PVForecastSolcastCommonSettings
 from akkudoktoreos.prediction.pvforecastvrm import PVForecastVrmCommonSettings
 
 
@@ -28,6 +29,7 @@ def pvforecast_provider_ids() -> list[str]:
             "PVForecastVrm",
             "PVForecastPVNode",
             "PVForecastForecastSolar",
+            "PVForecastSolcast",
         ]
 
     return [
@@ -197,6 +199,10 @@ class PVForecastCommonProviderSettings(SettingsBaseModel):
         default=None,
         json_schema_extra={"description": "PVForecastForecastSolar settings", "examples": [None]},
     )
+    PVForecastSolcast: Optional[PVForecastSolcastCommonSettings] = Field(
+        default=None,
+        json_schema_extra={"description": "PVForecastSolcast settings", "examples": [None]},
+    )
 
 
 class PVForecastCommonSettings(SettingsBaseModel):