structural refactor and codebase improvements

crates/application/src/users/resolve_username_to_id.rs

@@ -0,0 +1,23 @@
+//! Absorbs `handlers/users.rs::get_user_by_username`'s
+//! `repos.user.find_by_username()` call. `None` means "no such username" — the
+//! handler turns that into a 404, same as it always has; this is not an error
+//! condition.
+
+use domain::{
+    errors::DomainError,
+    value_objects::{UserId, Username},
+};
+
+use crate::users::deps::ResolveUsernameDeps;
+
+pub async fn execute(
+    deps: &ResolveUsernameDeps,
+    username: &Username,
+) -> Result<Option<UserId>, DomainError> {
+    let user = deps.user.find_by_username(username).await?;
+    Ok(user.map(|u| u.id().clone()))
+}
+
+#[cfg(test)]
+#[path = "tests/resolve_username_to_id.rs"]
+mod tests;