OPT-27: the night plan keeps the last fresh solar forecast

Forecast.Solar's sensors stop updating in the evening, so at night every such
site had no forecast: the night cheap run needed 1.5x the cycle cost and
Leftover solar planned nothing. One missed poll also dropped it at once.

solarfc.Hold keeps the last fresh reading of each sensor, for the day it is
for. A missing or stale sensor is covered by it for up to FRESH_S (a short
outage), and from the end of the bell (20:00) until local midnight + 3 h
however old. A tomorrow figure is tomorrow's until midnight and then today's;
a today figure is only today's. Stored in the controller's store
(solar_fc_hold), so an evening restart keeps it. The forecast line says
'Solar forecast read 17:05, used for tonight's plan.' (EN, NL).

test_tariff24_weather.py runs with a hold that never covers: it tests what the
plan does when the forecast IS lost.

Co-Authored-By: Claude <noreply@anthropic.com>

goodwe_controller/DOCS.md

@@ -555,6 +555,7 @@ With battery modes on, these choices replace the *Charging and selling on price*
 - The plan is checked every 15 minutes. A planned level moves only by 1 kWh or 10 %, so a wobbling forecast does not flip it.
 - No forecast, one older than 3 hours or from before midnight, or no house use yet: *Leftover solar* takes solar only and *Cheap hours only* fills to the top level. The Batteries screen says so.
 - A battery needs its capacity (`inverters.capacity_kwh`) for a plan.
+- **At night.** Some forecasts stop updating in the evening (Forecast.Solar does). The add-on keeps the last reading of each sensor and uses it for that night's plan: from 20:00 until 03:00, or until a newer reading comes. A sensor that is missing for up to 3 hours is covered the same way. A figure is never used for another day, and never older than that night. The Batteries screen says "Solar forecast read 17:05, used for tonight's plan." With no reading all day, the night run still needs the wider gap.
 
 **What always comes first.**
 - The peak limit, the safety stops and the monthly cycle come before a battery's mode.