Add PUT /users/:id endpoint

[SeikoCode]

Summary

• Adds an updateUser(id, { name, email }) helper to db/store.js that looks up a user by id and updates its name/email in place, mirroring the existing getUserById/createUser conventions.
• Adds a PUT /users/:id route in routes/users.js that validates the request body, returns 404 for unknown ids, and returns the updated user on success — following the same validation/response shape as the existing GET/POST /users handlers.

Why

tests/update-user.test.js (a grading test that must stay unedited) exercises a PUT /users/:id endpoint that didn't exist yet. This change implements it so the suite passes, without touching any other routes or the data-store's existing behavior.

Test plan

• npm test — all 3 update-user.test.js cases pass (update existing user → 200 with updated fields; unknown id → 404; missing field → 400), plus the existing users.test.js suite still passes
• npm run lint — no errors
• Manually: POST /users to create a user, then PUT /users/:id with a full body to confirm the response reflects the update; try a bogus id and a body missing email to confirm 404/400 respectively

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