Add updateUser store helper for PUT /users/:id

Finds a user by id and updates their name/email in place, returning
undefined when no user matches so the route layer can 404.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: 08smelnyk-collab

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 undefined;
+  }
+
+  user.name = name;
+  user.email = email;
+  return user;
+}
+
+module.exports = { getAllUsers, getUserById, createUser, updateUser };