You need to drop const...
You are just going to read the analog value which range from 0-1023, so declare your variable as int, which is 16-bit in Arduino, int16_t. Remember that an int in ESP-land is int32_t but once in string format on the Arduino, you will just send that via Tx and the ESP will have it on Rx as a string of digits.
Refer to the Firebase docs:
https://firebase.google.com/docs/reference/cpp