feat: add cloud PV forecast providers: pvnode.com, forecast-solar and solcast (#1150)

This PR adds three native PV power forecast providers, giving operators more
cloud forecast sources to choose from via pvforecast.provider, alongside the
existing PVForecastAkkudoktor, PVForecastVrm and PVForecastImport:

    PVForecastPVNode — native 15-minute forecasts from the pvnode.com
    V2 API. Saved-site mode (GET /v2/forecast/{site_id}) where the operator enters their API key +
    site id, or inline mode (POST /v2/forecast/inline) using the configured planes.
    PVForecastForecastSolar — the free Forecast.Solar API (no key
    required for the public endpoint). Multi-plane systems issue one request per plane and the
    instantaneous powers are summed per timestamp.
    PVForecastSolcast — the Solcast rooftop-site API (API key + resource id).

Implementation notes

    All three populate the existing pvforecast_ac_power (and mirror pvforecast_dc_power)
    prediction keys, so they slot into the optimizer unchanged.
    Timezone handling: pvnode and Forecast.Solar return site-local wall-clock timestamps with an
    IANA timezone field, which are resolved to absolute instants before resampling; Solcast returns
    UTC period_end and is normalised to the period start.
    Each provider follows the existing provider pattern (provider_id(), _request_forecast() with
    @cache_in_file, _update_data()), is registered in pvforecast.py and prediction.py, and is
    upstream-neutral.

Validation

    27 unit tests (timezone resolution, null/zero handling, kW→W and period-start conversion,
    azimuth conversion, multi-plane summation, request URL/auth, HTTP-error handling).
    ruff check (F/D/S/bandit) and ruff format clean.
    Each provider was additionally validated against its live API with a real plant, confirming
    the response shapes (pvnode: 288 native 15-min slots; Forecast.Solar: instantaneous watts;
    Solcast: kW estimates with period_end/PT30M).

Documentation

    Provider descriptions and configuration examples added to docs/akkudoktoreos/prediction.md.
    CHANGELOG.md entry under Unreleased.
    Regenerated docs/_generated/configpvforecast.md and configexample.md.

Notes for reviewers

Forecast.Solar's free endpoint is rate-limited (12 req/hour) and Solcast's free tier limits daily
calls; both providers rely on the standard 1-hour cache_in_file TTL to stay within budget.

Authors:

The code is created by Christin. Only minor adaptions by Bobby.

Signed-off-by: Christin <info@bikinibottom.capital>
Signed-off-by: Bobby Noelte <b0661n0e17e@gmail.com>
Co-authored-by: Christin <info@bikinibottom.capital>

docs/_generated/configexample.md

@@ -187,7 +187,10 @@
            "provider": "PVForecastAkkudoktor",
            "provider_settings": {
                "PVForecastImport": null,
-               "PVForecastVrm": null
+               "PVForecastVrm": null,
+               "PVForecastPVNode": null,
+               "PVForecastForecastSolar": null,
+               "PVForecastSolcast": null
            },
            "planes": [
                {