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) {

WeatherPredictor/web_server.cpp

@@ -27,41 +27,44 @@ static void handleCurrent() {
   doc["haveTrend"] = g_state.haveTrend;
   doc["forecast"]  = g_state.haveTrend ? g_state.forecast.text : "Collecting data...";
   doc["category"]  = g_state.haveTrend ? categoryShort(g_state.forecast.category) : "...";
+  doc["heap"]      = ESP.getFreeHeap();       // for monitoring memory health
   String out;
   serializeJson(doc, out);
   server.send(200, "application/json", out);
 }
 
+// Streamed (chunked) so we never build a large String/JsonDocument on the heap.
 static void handleHistory() {
-  JsonDocument doc;
+  server.setContentLength(CONTENT_LENGTH_UNKNOWN);
-  JsonArray arr = doc.to<JsonArray>();
+  server.send(200, "application/json", "");
+  server.sendContent("[");
+  char buf[80];
   int n = historyCount();
   for (int i = 0; i < n; i++) {
     Sample s = historyGet(i);
-    JsonObject o = arr.add<JsonObject>();
+    snprintf(buf, sizeof(buf), "%s{\"t\":%lu,\"msl\":%.1f,\"temp\":%.1f}",
-    o["t"]    = s.epoch;
+             i ? "," : "", (unsigned long)s.epoch, s.mslHpa, s.tempC);
-    o["msl"]  = s.mslHpa;
+    server.sendContent(buf);
-    o["temp"] = s.tempC;
   }
-  String out;
+  server.sendContent("]");
-  serializeJson(doc, out);
-  server.send(200, "application/json", out);
 }
 
+// Streamed (chunked). Forecast phrases contain no quotes/backslashes, so they
+// are safe to embed directly in JSON.
 static void handleForecasts() {
-  JsonDocument doc;
+  server.setContentLength(CONTENT_LENGTH_UNKNOWN);
-  JsonArray arr = doc.to<JsonArray>();
+  server.send(200, "application/json", "");
+  server.sendContent("[");
+  char buf[128];
   int n = flogCount();
-  for (int i = n - 1; i >= 0; i--) {          // newest first
+  for (int i = n - 1, first = 1; i >= 0; i--, first = 0) {   // newest first
     FcastEntry e = flogGet(i);
-    JsonObject o = arr.add<JsonObject>();
+    snprintf(buf, sizeof(buf), "%s{\"t\":%lu,\"cat\":\"%s\",\"text\":\"%s\"}",
-    o["t"]    = e.epoch;
+             first ? "" : ",", (unsigned long)e.epoch,
-    o["cat"]  = categoryShort((WxCategory)e.cat);
+             categoryShort((WxCategory)e.cat), forecastTextForLetter(e.letter));
-    o["text"] = forecastTextForLetter(e.letter);
+    server.sendContent(buf);
   }
-  String out;
+  server.sendContent("]");
-  serializeJson(doc, out);
-  server.send(200, "application/json", out);
 }
 
 static void handleGetSettings() {