feat: DS3231 RTC read/set with lost-power detection

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

WeatherPredictor/rtc_time.cpp

@@ -0,0 +1,20 @@
+#include <RTClib.h>
+#include "rtc_time.h"
+
+static RTC_DS3231 rtc;
+
+bool rtcBegin()      { return rtc.begin(); }
+bool rtcLostPower()  { return rtc.lostPower(); }
+
+RtcTime rtcNow() {
+  DateTime n = rtc.now();
+  return RtcTime{ n.year(), n.month(), n.day(), n.hour(), n.minute(), n.second() };
+}
+
+void rtcSet(const RtcTime& t) {
+  rtc.adjust(DateTime(t.year, t.month, t.day, t.hour, t.minute, t.second));
+}
+
+uint32_t rtcEpoch() {
+  return rtc.now().unixtime();
+}