feat(MQTT): 协议 V1.13 — initialize extra_info 增加 imei/iccid 可选字段

用户需求 2026-08-31:
- 协议 §5.1: extra_info 增加 imei(4G 模块 IMEI)/ iccid(流量卡 ICCID)可选字段
- 语义: 无 4G 模块省略/空串;4G 通道由 Air780 填真实值(§6.2)
- 固件 iot_send_initialize: extra_info 补 imei/iccid(空串占位, DBN 无 4G 信息;
  待"BLE→Air780 配置同步"实现后可回读填充)
- 同步: README 索引 V1.13 + devlog 置顶条目

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/iot_mqtt_srv.c

@@ -1085,7 +1085,7 @@ static void iot_send_initialize(void) {
         "\"data\":{\"dev_serial\":\"%s\",\"model\":\"%s\","
         "\"hard_ver\":\"%s\",\"soft_ver\":\"%s\","
         "\"loop_ver\":\"%s\",\"loop_hw_ver\":\"%s\","
-        "\"extra_info\":{\"code\":\"0\",\"csq\":\"0\",\"location\":\"\"}}}",
+        "\"extra_info\":{\"code\":\"0\",\"imei\":\"\",\"iccid\":\"\",\"csq\":\"0\",\"location\":\"\"}}}",
         (unsigned long)(g_iot_msg_id + 1), dev_time_now(),
         g_iot_dev_serial, PRODUCT_MODEL, HARDWARE_VER, FIRMWARE_VER,
         loop_ver_str, loop_hw_ver_str);