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

[teovaira]

Summary

• Adds PUT /users/:id to update an existing user's name and email
• Adds an updateUser helper in db/store.js that the new route calls
• Follows the same validation/not-found patterns already used by POST /users and GET /users/:id

Test plan

• npm test passes (9/9)
• PUT /users/:id with a valid name+email on an existing user returns 200 with the updated fields
• PUT /users/:id on a non-existent id returns 404
• PUT /users/:id with a missing name or email returns 400
• Existing GET/POST/health routes are unaffected

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