Add REST API endpoints for active resource overview

Add REST API endpoints to query active resource overview per domain and project.

Reference pattern: api/rest/resource_slot/handler.py and api/rest/domain/handler.py

## Scope

### Endpoints
- GET /v8/domains/{domain_name}/resource-overview
- GET /v8/projects/{project_id}/resource-overview

### DTO (common/dto/manager/resource_slot/response.py)
- ActiveResourceOverviewResponse: BaseRootResponseModel
- slots: list of {slot_name: str, quantity: str (decimal string)}
- session_count: int

### Handler (api/rest/resource_slot/handler.py or api/rest/domain/handler.py)
- get_domain_resource_overview: PathParam[domain_name] -> ActiveResourceOverviewResponse
- get_project_resource_overview: PathParam[project_id] -> ActiveResourceOverviewResponse
- Calls Processor via info.context

### Registry
- Register new routes in appropriate registry.py

## Success Criteria
- [ ] GET /v8/domains/{domain_name}/resource-overview returns 200 with slot list and session_count
- [ ] GET /v8/projects/{project_id}/resource-overview returns 200 with slot list and session_count
- [ ] Returns empty slots and session_count=0 when no active sessions
- [ ] Non-existent domain/project returns appropriate error
- [ ] pants check passes for affected packages

JIRA Issue: BA-4917