fix: stream large API responses to stop heap-exhaustion truncation

Serving /api/history (and /api/forecasts) built a ~12 KB String +
JsonDocument every 15 s; over days this fragmented the ESP8266 heap until
the TCP stack could no longer send a full response, causing the web page
to fail with ERR_CONTENT_LENGTH_MISMATCH. Stream both array endpoints via
chunked transfer (small stack buffer, no big allocations). Add free-heap
to /api/current and Serial, plus a low-memory safety restart (history is
persisted first).

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

WeatherPredictor/WeatherPredictor.ino

@@ -88,6 +88,15 @@ void loop() {
     tSample = now;
     sampleNow();
     displayRender(g_state);
+    Serial.printf("heap=%u maxblock=%u\n", ESP.getFreeHeap(), ESP.getMaxFreeBlockSize());
+    // Safety net: if the largest contiguous block gets too small, the TCP stack
+    // can no longer send responses. Persist and reboot to defragment.
+    if (ESP.getMaxFreeBlockSize() < 6000) {
+      Serial.println(F("Low memory -> saving history and restarting"));
+      historySave();
+      delay(50);
+      ESP.restart();
+    }
   }
 
   if (now - tHistory >= HISTORY_INTERVAL_MS) {