feat: add EnergyCharts feed-in tariff provider (#1165)

The `FeedInTariffEnergyCharts` provider uses the raw Energy-Charts day-ahead market price as the
feed-in tariff. It stores prices in `feed_in_tariff_wh` without adding electricity import charges
or VAT. The data is loaded from the Energy-Charts `/price` endpoint for the configured bidding
zone. The native Energy-Charts resolution, including quarter-hour data, is retained.

Energy-Charts usually supplies prices only for the published day-ahead period. If that data does
not cover the complete configured prediction horizon, the provider extends it as follows:

- With more than 800 hours of history, an ETS (Holt-Winters exponential smoothing) forecast with
  weekly seasonality is used.
- With more than 168 hours of history, an ETS forecast with daily seasonality is used.
- With less history, the median of the available values is used as a constant fallback.

The seasonal periods are adjusted to the source resolution. For example, quarter-hour data uses
four values per hour. Values already supplied by Energy-Charts are kept unchanged; only missing
future slots after the last published price are forecast. Consequently, a 15-minute optimization
uses four forecast values per hour without converting them to hourly averages.

Signed-off-by: Andreas Schmitz <akkudoktor.net>
Signed-off-by: Bobby Noelte <b0661n0e17e@gmail.com>

tests/testdata/docs/_generated/configexample.md

@@ -129,6 +129,9 @@
            "feedintariffimport": {
                "import_file_path": null,
                "import_json": null
+           },
+           "energycharts": {
+               "bidding_zone": "DE-LU"
            }
        },
        "general": {