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/flog.h

@@ -0,0 +1,16 @@
+#pragma once
+#include <Arduino.h>
+#include "forecast.h"
+
+struct FcastEntry {
+  uint32_t epoch;
+  char     letter;   // Zambretti 'A'..'Z'
+  uint8_t  cat;      // WxCategory
+};
+
+void       flogBegin();
+void       flogConsider(uint32_t epoch, const Forecast& f);  // logs only when it changes
+int        flogCount();
+FcastEntry flogGet(int i);   // 0 = oldest
+bool       flogSave();
+bool       flogLoad();