Add PUT /users/:id (update a user) - #84

Summary

• Add updateUser(id, {name, email}) to db/store.js
• Add PUT /users/:id in routes/users.js: validates name/email are present (400), returns 404 for an unknown id, otherwise updates and returns the user (200)
• Fill in NOTES.md per the project's grading checklist

Test plan

• npm test — 9/9 passing, including all three tests/update-user.test.js cases (update, 404, 400)
• npm run lint — clean
• Manual check: PUT /users/1 with a partial body returns 400; PUT /users/9999 with a full body returns 404; PUT /users/1 with a full body returns 200 with the updated user

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