refine the user management API

### 🚀 Feature Description and Motivation

https://github.com/vllm-project/aibrix/pull/1639#discussion_r2417964859

The current user management API (/CreateUser, /ReadUser, etc., all using POST) is a direct migration from the previous Go service. While functional, it doesn't follow standard RESTful API design principles, which can make it less intuitive and harder for new developers to use.

Since this is a new Python service, it's a great opportunity to adopt a more standard RESTful approach. I suggest refactoring these endpoints to use standard HTTP methods and resource-based paths. For example:

- Create User: POST /v1/users
- Read User: GET /v1/users/{user_name}
- Update User: PUT /v1/users/{user_name}
- Delete User: DELETE /v1/users/{user_name}

This change would improve the API's clarity, predictability, and align it with modern web service best practices.

### Use Case

user management for rate limits etc

### Proposed Solution

_No response_

Start by locating the current /CreateUser, /ReadUser, and related user-management endpoints in the Python service, then review the linked pull-request discussion. Done means exposing user resources under /v1/users with POST, GET, PUT, and DELETE as described, while preserving the user-management behavior for rate limits.

Written by the indexing model from the issue text.