Controller: Aggregate worker topology status into Slice status (Partial Mesh MVP)

## Summary
Expose Hub-and-Spoke topology readiness at the Slice level by aggregating worker-reported status.

## Desired Slice status (MVP)
- Selected hubs (names)
- Derived spokes count (or list)
- Readiness summary:
- desiredEdges: N
- readyEdges: M
- failingEdges: K
- Basic failure reasons for failing edges (best-effort)

## Inputs
Worker-operator should report per-peer connectivity status (see worker issue). Controller should consume that and present a summarized view.

## Acceptance Criteria
- Slice status reflects topology progress and failures
- Status updates on topology changes (hub/spoke changes)
- Minimal but actionable (operators can see what's failing and where)

## Tasks
- [ ] Define minimal status fields for MVP
- [ ] Implement aggregation
- [ ] Add tests for aggregation logic