Add PUT /users/:id endpoint to update a user

NOTES.md

@@ -0,0 +1,14 @@
+Note: `routes/users.js` is included in the committed changes.
+
+- What was in the plan you approved, and did you edit anything before approving?
+
+The plan was to add an `updateUser` helper to `db/store.js` that finds a user by id and returns null if it doesn't exist, then add a `PUT /users/:id` route in `routes/users.js` that validates `name` and `email` are present (400 if not), looks up the user via the store (404 if missing), and otherwise updates and returns it. I didn't need to edit anything — it matched the existing GET/POST patterns closely enough to approve as-is.
+
+- Which model did you choose, and why?
+I chose Sonnet as my model, as we are making changes where there is a clear pattern of code to follow from.
+
+- How did you split your commits, and why that way?
+One commit for the `db/store.js` change (the `updateUser` helper) and a second for the `routes/users.js` route, since they're separate layers of the same feature and each is reviewable on its own. NOTES.md is its own commit, and a small follow-up commit added intentional whitespace to `routes/users.js`.
+
+- What did your review catch — or confirm was already fine?
+Review confirmed validation runs before the not-found check (matching the existing POST behavior) and that the not-found response returns a clean 404 instead of throwing. No changes were needed.

db/store.js

@@ -24,4 +24,16 @@ function createUser({ name, email }) {
   return user;
 }
 
-module.exports = { getAllUsers, getUserById, createUser };
+function updateUser(id, { name, email }) {
+  const user = getUserById(id);
+
+  if (!user) {
+    return null;
+  }
+
+  user.name = name;
+  user.email = email;
+  return user;
+}
+
+module.exports = { getAllUsers, getUserById, createUser, updateUser };

routes/users.js

@@ -32,4 +32,23 @@ router.post("/", (req, res) => {
   res.status(201).json(user);
 });
 
+// PUT /users/:id — update an existing user; name and email are required
+router.put("/:id", (req, res) => {
+  const { name, email } = req.body;
+
+  if (!name || !email) {
+    return res.status(400).json({ error: "name and email are required" });
+  }
+
+  const id = Number(req.params.id);
+  const user = store.updateUser(id, { name, email });
+
+  if (!user) {
+    return res.status(404).json({ error: "User not found" });
+  }
+
+  res.json(user);
+
+});
+
 module.exports = router;