feat: NTP time sync writing to DS3231 (by IP, DNS-independent)

This LAN's router does not resolve external hostnames, so NTP uses
hard-coded Cloudflare/Google anycast IPs instead of names.

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

WeatherPredictor/rtc_time.h

@@ -11,3 +11,4 @@ bool     rtcLostPower();
 RtcTime  rtcNow();
 void     rtcSet(const RtcTime& t);
 uint32_t rtcEpoch();
+bool     ntpSync(int tzOffsetMin);   // true if synced and RTC updated