Add PUT /users/:id endpoint

[huuphong91]

Summary

• Add PUT /users/:id to update an existing user's name and email
• Add updateUser(id, { name, email }) to db/store.js, following the existing helper pattern
• Validate name and email are present (400 on a missing field, matching the existing POST /users rule)
• Return 404 when the id doesn't exist instead of crashing
• Add NOTES.md covering the plan, model choice, commit split, and self-review

Test plan

• npm test passes locally (9/9, including the provided tests/update-user.test.js and tests/notes.test.js)
• PUT /users/:id with a valid body updates and returns the user (200)
• PUT /users/:id for a nonexistent id returns 404
• PUT /users/:id with a missing name or email returns 400