Implement Admin GraphQL API for service configuration

## Overview

Create GraphQL API endpoints for superadmins to manage service configurations.

## Tasks

### GraphQL Schema

- `ServiceConfigNode` type with fields: `key`, `value`, `description`, `updatedAt`
- Query: `service_configs`, `service_config(key: String!)`
- Mutations: `create_service_config`, `update_service_config`, `delete_service_config`

### Authorization

- Superadmin-only access for all operations
- Read access potentially for admins (configurable)

### Input Validation

- Key format validation
- Value type validation based on config key

## Files to Create/Modify

- `src/ai/backend/manager/api/gql/service_config.py`
- Update GraphQL schema root

JIRA Issue: BA-3761