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/WeatherPredictor.ino

@@ -6,6 +6,7 @@
 #include "display_ui.h"
 #include "forecast.h"
 #include "history.h"
+#include "flog.h"
 #include "app_settings.h"
 #include "net.h"
 #include "web_server.h"
@@ -28,6 +29,7 @@ static void sampleNow() {
   if (g_state.haveTrend) {
     g_state.trend    = classifyTrend(d);
     g_state.forecast = computeForecast(g_state.mslHpa, g_state.trend, g_state.now.month);
+    flogConsider(rtcEpoch(), g_state.forecast);   // log only when the forecast changes
   } else {
     g_state.trend = TREND_STEADY;
   }
@@ -52,6 +54,9 @@ void setup() {
   historyBegin();
   historyLoad();   // restore prior samples (empty on first ever boot)
   Serial.printf("history restored: %d samples\n", historyCount());
+  flogBegin();
+  flogLoad();      // restore forecast change log
+  Serial.printf("forecast log restored: %d entries\n", flogCount());
 
   displaySplash("WiFi", "Connect / portal");
   if (netBegin()) {