feat: add PUT /users/:id route

Wires the update-a-user endpoint into the users resource. It rejects
a missing name or email with 400 (same check as POST /users), coerces
the id param to a number to match the store's strict id comparison,
returns 404 when store.updateUser finds no such user, and responds
with the updated user on success.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019LQYyxNEbgzmiBWqaq8o9f

Author: RFrams

routes/users.js

@@ -32,4 +32,22 @@ 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;