10.User CRUD API — Part 1: Create, Update, Delete & Email Uniqueness

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You're building the core of a User Management REST API using Express. There's no real database — user data is stored in an in-memory array that acts as a mock DB for this exercise.

The API should let clients create, update, and delete users, while enforcing email uniqueness across all users.

Endpoints to implement

Method	Route	Description
GET	`/`	Health check
POST	`/users`	Create a new user
GET	`/users/:id`	Fetch a single user by id
PUT	`/users/:id`	Update a user's name and/or email
DELETE	`/users/:id`	Delete a user

Business rules

Create (POST /users)
- name and email are required in the request body.
- If either is missing, respond with 400 and { "message": "Name and email are required" }.
- If the email is already registered by another user, respond with 409 and { "message": "Email already registered" }.
- On success, respond with 201 and the newly created user object (id, name, email).
Read (GET /users/:id)
- Return the matching user with 200.
- If no user exists with that id, respond with 404 and { "message": "User not found" }.
Update (PUT /users/:id)
- Accepts name and/or email in the body — only provided fields should be updated.
- If the user doesn't exist, respond with 404.
- If the new email is already taken by a different user, respond with 409. Updating a user with their own current email (i.e. no real change) must not be treated as a conflict.
- On success, respond with 200 and the updated user object.
Delete (DELETE /users/:id)
- Removes the user from the store and responds with 204 (no body).
- If the user doesn't exist, respond with 404.

Notes

No database, no auth, no persistence — everything lives in memory for the lifetime of the process.
Email comparisons should be treated as case-insensitive.

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