OAuth2 login endpoint. Initiates the OAuth2 authorization code flow.

**Query Parameters:**
- `redirect_uri`: The URI to redirect to after authentication (required)
- `state`: A random string for CSRF protection (optional but recommended)

**Flow:**
1. Client redirects user to this endpoint with `redirect_uri` and `state`
2. This endpoint redirects user to Auth0 for authentication
3. After authentication, Auth0 redirects to `/callback` with authorization code
4. `/callback` redirects back to the original `redirect_uri` with code and state

**Example:**
```
GET /login?redirect_uri=https://chat.openai.com&state=abc123
```