refactor(backend): Use more efficient bytes to string methods and remove unnecessary type conversions

detail:
    1. add utils.BytesToString function instead of string() convert bytes to string.
    2. use bytes.NewReader instead of strings.NewReader(string(sb)).
    3. use w.Body.Bytes() instead of []byte(w.Body.String()).

backend/utils/helpers.go

@@ -0,0 +1,7 @@
+package utils
+
+import "unsafe"
+
+func BytesToString(b []byte) string {
+	return *(*string)(unsafe.Pointer(&b))
+}