totp: generate backup code, attempt to use backup code when checking totp, delete backup after successful use

crates/hash/src/secret.rs

@@ -1,7 +1,7 @@
 use crate::error::Error;
 use crate::hash::{hash, Hash};
 use nanoid::nanoid;
-use nanoid_dictionary::ALPHANUMERIC;
+use nanoid_dictionary::{ALPHANUMERIC, NUMBERS};
 use std::fmt::{Display, Formatter};
 
 // Struct to generate the secret
@@ -10,6 +10,9 @@ impl SecretString {
     pub fn new(length: usize) -> Self {
         Self(nanoid!(length, ALPHANUMERIC))
     }
+    pub fn new_digits(length: usize) -> Self {
+        Self(nanoid!(length, NUMBERS))
+    }
 }
 impl Default for SecretString {
     fn default() -> Self {