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/web_server.cpp

@@ -3,6 +3,7 @@
 #include "state.h"
 #include "app_settings.h"
 #include "history.h"
+#include "flog.h"
 #include "forecast.h"
 #include "web_page.h"
 
@@ -47,6 +48,22 @@ static void handleHistory() {
   server.send(200, "application/json", out);
 }
 
+static void handleForecasts() {
+  JsonDocument doc;
+  JsonArray arr = doc.to<JsonArray>();
+  int n = flogCount();
+  for (int i = n - 1; i >= 0; i--) {          // newest first
+    FcastEntry e = flogGet(i);
+    JsonObject o = arr.add<JsonObject>();
+    o["t"]    = e.epoch;
+    o["cat"]  = categoryShort((WxCategory)e.cat);
+    o["text"] = forecastTextForLetter(e.letter);
+  }
+  String out;
+  serializeJson(doc, out);
+  server.send(200, "application/json", out);
+}
+
 static void handleGetSettings() {
   JsonDocument doc;
   doc["altitude"] = settings.altitudeM;
@@ -76,6 +93,7 @@ void webBegin() {
   server.on("/", handleRoot);
   server.on("/api/current", handleCurrent);
   server.on("/api/history", handleHistory);
+  server.on("/api/forecasts", handleForecasts);
   server.on("/api/settings", HTTP_GET,  handleGetSettings);
   server.on("/api/settings", HTTP_POST, handlePostSettings);
   server.begin();