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/WeatherPredictor.ino

@@ -55,6 +55,10 @@ void setup() {
   displaySplash("WiFi", "Connect / portal");
   if (netBegin()) {
     Serial.printf("WiFi connected: %s\n", netIP().c_str());
+    if (ntpSync(settings.tzOffsetMin))
+      Serial.println(F("RTC synced from NTP"));
+    else
+      Serial.println(F("NTP sync failed"));
   } else {
     Serial.println(F("WiFi not connected - running offline"));
   }

WeatherPredictor/config.h

@@ -30,5 +30,5 @@ static const float DEFAULT_LAT           = 54.9870f;
 static const float DEFAULT_LON           = 82.8730f;
 
 // ---- Network ----
-#define NTP_SERVER "pool.ntp.org"
+// NTP is done by hard-coded IP in rtc_time.cpp (this LAN's DNS is unreliable).
 #define AP_NAME    "WeatherPredictor-Setup"

WeatherPredictor/rtc_time.cpp

@@ -1,4 +1,7 @@
 #include <RTClib.h>
+#include <time.h>
+#include <ESP8266WiFi.h>
+#include "config.h"
 #include "rtc_time.h"
 
 static RTC_DS3231 rtc;
@@ -18,3 +21,22 @@ void rtcSet(const RtcTime& t) {
 uint32_t rtcEpoch() {
   return rtc.now().unixtime();
 }
+
+bool ntpSync(int tzOffsetMin) {
+  // NTP servers by IP to bypass DNS (this network doesn't resolve names reliably).
+  // Cloudflare (162.159.200.123) and Google (216.239.35.0/.4) are stable anycast.
+  configTime(tzOffsetMin * 60, 0, "162.159.200.123", "216.239.35.0", "216.239.35.4");
+  time_t now = time(nullptr);
+  int tries = 0;
+  while (now < 1700000000 && tries < 120) {        // up to ~30 s
+    delay(250);
+    now = time(nullptr);
+    tries++;
+  }
+  if (now < 1700000000) return false;
+  struct tm* lt = localtime(&now);
+  rtcSet(RtcTime{ (uint16_t)(lt->tm_year + 1900), (uint8_t)(lt->tm_mon + 1),
+                  (uint8_t)lt->tm_mday, (uint8_t)lt->tm_hour,
+                  (uint8_t)lt->tm_min, (uint8_t)lt->tm_sec });
+  return true;
+}

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