Feature Request: Add API endpoint to query account credits balance

## Feature Request

**Summary:**
Please add an API endpoint to retrieve the current user's credits balance programmatically.

## Current Behavior

The existing `GET /api/user` endpoint only returns the account status (`active` or `waitlisted`).
There is no API endpoint available to query credits balance. The only way to check credits
is by manually logging into https://platform.comfy.org/profile.

## Desired Behavior

A new endpoint (suggested):

GET /api/user/credits

Expected response:
{
"credits": 123.45,
"currency": "USD"
}

Or alternatively, include credits info in the existing `GET /api/user` response.

## Use Case

- Programmatic monitoring of remaining credits
- Automated alerts when credits fall below a threshold
- Integration into custom dashboards or billing workflows

## References

- Cloud API Docs: https://docs.comfy.org/zh/api-reference/cloud/user/get-current-user-information