feat: add validation for username and password length in user creation

crates/backend/src/controller/user.rs

@@ -12,8 +12,10 @@ pub fn setup(cfg: &mut actix_web::web::ServiceConfig) {
 
 #[derive(Deserialize, Validate)]
 struct CreateUser {
+    #[validate(length(min = 4))]
     username: String,
     name: String,
+    #[validate(length(min = 8))]
     password: String,
 }