fix: UUID fallback in GET /users/{id} so AP actor URLs resolve for signature verification

crates/presentation/src/handlers/users.rs

@@ -32,7 +32,16 @@ pub async fn get_user(
     OptionalAuthUser(viewer): OptionalAuthUser,
     headers: HeaderMap,
 ) -> Result<Response, ApiError> {
-    let user = get_user_by_username(&*s.users, &username).await?;
+    // AP actor URLs use the user's UUID (e.g. /users/{uuid}). Fall back to UUID lookup
+    // so remote servers can fetch the actor JSON for HTTP signature verification.
+    let user = if let Ok(uuid) = uuid::Uuid::parse_str(&username) {
+        s.users
+            .find_by_id(&domain::value_objects::UserId::from_uuid(uuid))
+            .await?
+            .ok_or(ApiError::Domain(domain::errors::DomainError::NotFound))?
+    } else {
+        get_user_by_username(&*s.users, &username).await?
+    };
 
     let accept = headers
         .get(header::ACCEPT)