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/app/homeview.py

@@ -543,7 +543,8 @@ def solar_block(c, now):
         out["line"] = _t("solar." + st)
     elif fc is not None:
         out["line"] = line_solar(fc.today.kwh, out["count_kwh"],
-                                 out["updated"])
+                                 out["updated"],
+                                 kept=getattr(fc, "kept", False))
     return out
 
 
@@ -1395,7 +1396,11 @@ def day_note(key, **values) -> str:
     return _t("day." + key, **values)
 
 
-def line_solar(fc_kwh, count_kwh, at) -> str:
+def line_solar(fc_kwh, count_kwh, at, kept=False) -> str:
+    """The forecast line. OPT-27: a reading kept past three hours (the
+    evening's, used at night) says so instead of "Updated"."""
+    if kept:
+        return _t("solar.kept", at=at)
     return _t("solar.line", kwh=_kwh(fc_kwh), count=_kwh(count_kwh), at=at)