refactor(application): fix 4 code smells — validate username input, extract ownership guard and dedup helpers

crates/application/src/use_cases/profile.rs

@@ -10,7 +10,7 @@ pub async fn get_user(users: &dyn UserRepository, user_id: &UserId) -> Result<Us
 }
 
 pub async fn get_user_by_username(users: &dyn UserRepository, username: &str) -> Result<User, DomainError> {
-    let username = Username::from_trusted(username.to_string());
+    let username = Username::new(username).map_err(|_| DomainError::NotFound)?;
     users.find_by_username(&username).await?.ok_or(DomainError::NotFound)
 }