feat: forecast change log

Record each forecast change (Zambretti letter transition) with a
timestamp into a 24-entry ring buffer, persisted to LittleFS and
restored on boot. New /api/forecasts endpoint and a 'Forecast log'
panel in the web UI (newest first, category colour, timestamp).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

WeatherPredictor/forecast.cpp

@@ -88,6 +88,11 @@ Forecast computeForecast(float p, Trend trend, int month) {
   return Forecast{ letter, ZTEXT[letter - 'A'], categoryOf(letter) };
 }
 
+const char* forecastTextForLetter(char letter) {
+  if (letter < 'A' || letter > 'Z') return "";
+  return ZTEXT[letter - 'A'];
+}
+
 const char* categoryShort(WxCategory c) {
   switch (c) {
     case WX_FINE:       return "Fine";