fix: price interpolation (#1154)

Use forward fill to interpolate time series data that represents prices:
- elecprice_marketprice_wh
- feed_in_tariff_wh

Signed-off-by: Bobby Noelte <b0661n0e17e@gmail.com>

src/akkudoktoreos/server/eos.py

@@ -1203,6 +1203,7 @@ async def fastapi_strompreis() -> list[float]:
             key="elecprice_marketprice_wh",
             start_datetime=start_datetime,
             end_datetime=end_datetime,
+            fill_method="ffill",
         )
         elecprice_list = elecprice_array.tolist()
     except Exception as e: