code: static String autoId() {
      StringBuilder builder = new StringBuilder();
      int maxRandom = AUTO_ID_ALPHABET.length();
      for (int i = 0; i < AUTO_ID_LENGTH; i++) {
        builder.append(AUTO_ID_ALPHABET.charAt(RANDOM.nextInt(maxRandom)));
      }
      return builder.toString();
    }
docs: Creates a pseudo-random 20-character ID that can be used for Firestore documents.