feat(pvforecast): add calibrated local Akkudoktor backend

Port local PV modeling and outage calibration from feature commits f976335, 6dc58c3 and faed0fd by Andreas. Keep PVForecastAkkudoktor identity and remote default, adapt to async storage, and migrate legacy provider settings.

docs/akkudoktoreos/prediction.md

@@ -839,6 +839,62 @@ Example:
 }
 ```
 
+#### Local Open-Meteo/pvlib backend
+
+`PVForecastAkkudoktor` keeps the remote API as its default backend. To calculate
+PV output locally, keep the same public provider ID and set:
+
+```json
+{
+  "pvforecast": {
+    "provider": "PVForecastAkkudoktor",
+    "akkudoktor": {
+      "backend": "local",
+      "resolution_minutes": 15,
+      "calibration_enabled": false
+    }
+  }
+}
+```
+
+Configure `general.latitude`, `general.longitude` and `pvforecast.planes` as
+usual. The local PVWatts model uses plane peak power, tilt, azimuth, horizon,
+tracking, mounting, losses and inverter power. It does not use the CEC module
+and inverter names required by `PVForecastPVLib`; that provider remains a
+separate option backed by the selected weather provider.
+
+The local backend requests irradiance and weather directly from Open-Meteo,
+then computes DC and AC power with pvlib. Both prediction keys retain their
+units of **W**. Fifteen-minute power samples represent 0.25 hours when converted
+to Wh by an optimizer. Hourly consumers obtain the hourly mean power. Neither
+backend selects or changes the optimization algorithm or the legacy `/optimize`
+request schema.
+
+Open-Meteo radiation timestamps label the preceding interval. By default EOS
+shifts them to interval starts and uses the interval midpoint for solar position.
+Native 15-minute weather is available in Central Europe and North America;
+elsewhere Open-Meteo interpolates hourly weather. See the
+[Open-Meteo API documentation](https://open-meteo.com/en/docs).
+
+Set `calibration_enabled` to true and configure cumulative PV production meter
+keys in `measurement.pv_production_emr_keys` to fit the model against measurements.
+Calibration preserves daily energy while optionally correcting the intraday
+azimuth shape, clamps correction factors and respects inverter capacity.
+Hourly meters are fitted hourly; native quarter-hour meters retain their cadence.
+Probable outage or curtailment days are excluded by default. Without sufficient
+usable measurements the uncalibrated physical model is used.
+
+Transient weather errors are retried. If an update still fails, existing usable
+AC forecasts are retained; a cold start or expired-only forecast reports the
+error. Retention does not extend the available forecast horizon. Calibration and
+forecast settings are listed in the generated PV configuration reference.
+
+Feature-branch configurations using `PVForecastAkkudoktorLocal` and
+`provider_settings.PVForecastAkkudoktorLocal` migrate to the public provider ID
+and `akkudoktor.backend = "local"`. Explicit current `akkudoktor` values take
+precedence over legacy values. Invalid local settings fail validation instead
+of being silently dropped during file migration.
+
 ### PVForecastVrm Provider
 
 The `PVForecastVrm` provider retrieves pv power forecast data from the Victron Remote Management