Add user-level app config GraphQL query

## Description

Implement GraphQL query to retrieve app configuration at the user level with full hierarchy inheritance (domain → project → user).

## Tasks

- Define user-level app config schema
- Implement query resolver with full inheritance chain
- Add permission checks
- Write tests including all inheritance scenarios

## Acceptance Criteria

- User-level app config can be queried via GraphQL
- Project and domain config inheritance works correctly
- Users can only query their own configs
- Tests verify functionality and full inheritance chain

JIRA Issue: BA-4124