CUB-230: ESP32 offline status buffering and replay on reconnect

- Add SPIFFS buffer for status entries when MQTT offline
- FIFO eviction when max_buffer_entries exceeded
- Replay buffer on MQTT reconnect
- Configurable buffer size in esp32-config.json

firmware/.pio/libdeps/esp32-mqtt/PubSubClient/examples/mqtt_reconnect_nonblocking/mqtt_reconnect_nonblocking.ino

@@ -0,0 +1,67 @@
+/*
+ Reconnecting MQTT example - non-blocking
+
+ This sketch demonstrates how to keep the client connected
+ using a non-blocking reconnect function. If the client loses
+ its connection, it attempts to reconnect every 5 seconds
+ without blocking the main loop.
+
+*/
+
+#include <SPI.h>
+#include <Ethernet.h>
+#include <PubSubClient.h>
+
+// Update these with values suitable for your hardware/network.
+byte mac[]    = {  0xDE, 0xED, 0xBA, 0xFE, 0xFE, 0xED };
+IPAddress ip(172, 16, 0, 100);
+IPAddress server(172, 16, 0, 2);
+
+void callback(char* topic, byte* payload, unsigned int length) {
+  // handle message arrived
+}
+
+EthernetClient ethClient;
+PubSubClient client(ethClient);
+
+long lastReconnectAttempt = 0;
+
+boolean reconnect() {
+  if (client.connect("arduinoClient")) {
+    // Once connected, publish an announcement...
+    client.publish("outTopic","hello world");
+    // ... and resubscribe
+    client.subscribe("inTopic");
+  }
+  return client.connected();
+}
+
+void setup()
+{
+  client.setServer(server, 1883);
+  client.setCallback(callback);
+
+  Ethernet.begin(mac, ip);
+  delay(1500);
+  lastReconnectAttempt = 0;
+}
+
+
+void loop()
+{
+  if (!client.connected()) {
+    long now = millis();
+    if (now - lastReconnectAttempt > 5000) {
+      lastReconnectAttempt = now;
+      // Attempt to reconnect
+      if (reconnect()) {
+        lastReconnectAttempt = 0;
+      }
+    }
+  } else {
+    // Client connected
+
+    client.loop();
+  }
+
+}