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

[darioduarte1]

Summary

• Adds PUT /users/:id to update an existing user's name/email
• Validates input (400 when a field is missing) and returns 404 when the id doesn't exist, following the patterns already used by GET /:id and POST /
• Adds a matching updateUser helper in db/store.js

Test plan

• npm test passes (9/9), including the pre-written tests/update-user.test.js (update, 404, 400 cases)
• NOTES.md documents the plan, model choice, commit split, and self-review
• To test manually: PUT /users/1 with a full body returns 200 with the updated user; PUT /users/9999 returns 404; PUT /users/1 with a missing field returns 400

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