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

[julien-vidal]

Summary

• Add an updateUser(id, { name, email }) helper in db/store.js, matching the shape of the existing getUserById/createUser helpers; returns undefined for an unknown id.
• Add PUT /users/:id in routes/users.js: validates name (non-empty string) and email (basic shape, whitespace trimmed before both validation and storage) before the id lookup, then 404s if the user doesn't exist, or returns the updated user with 200.
• Add tests/update-user.edge-cases.test.js covering cases the fixed grading file (tests/update-user.test.js, left untouched) doesn't: non-numeric id, malformed/whitespace-padded email, empty body, whitespace-only name, non-string fields, that the update actually persists to the store, and that the id is never reassigned.
• Add NOTES.md with the plan, model choice, commit rationale, and self-review notes (including a bug caught during review: email validation ran a regex against the untrimmed value while storage trimmed it, so a valid email with incidental surrounding whitespace was wrongly rejected — fixed by trimming before validating).

Test plan

• npm test — 18/18 passing, including all three tests in tests/update-user.test.js
• npm run lint — clean
• Reviewer: confirm PUT /users/1 with a valid body returns 200 and the updated fields
• Reviewer: confirm PUT /users/9999 returns 404
• Reviewer: confirm PUT /users/1 with a missing/invalid field returns 400

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