Add PUT /users/:id endpoint to update users

[barnzdan]

Summary

Added a PUT /users/:id endpoint to the users resource that updates an existing user by id. The change includes an updateUser helper function in the store layer and full input validation with proper error responses.

What changed

• db/store.js: Added updateUser(id, { name, email }) function that updates a user in place and returns the updated user or undefined if not found.
• routes/users.js: Added PUT /:id handler that validates required fields (name and email), calls the store function, and returns appropriate status codes (200 on success, 400 on validation failure, 404 if user not found).
• NOTES.md: Added implementation notes documenting the plan, model choice, commit strategy, and review findings.

Testing

• All grading tests pass: update existing user (200), update non-existent user (404), missing required field (400).
• All existing tests remain green: health check, list users, get user by id, create user.
• Linter is clean.

Test cases to verify

• Success case: PUT /users/1 with {"name": "Updated", "email": "updated@example.com"} should return 200 with updated fields.
• Not found: PUT /users/9999 with valid data should return 404.
• Invalid input: PUT /users/1 with {"name": "Only Name"} (missing email) should return 400.