P1: Implement firewall_status_summary (replace placeholder) with safe read-only output

http_api.py contains a TODO placeholder for firewall_status_summary. Replace with real implementation.

Acceptance:
- Read-only fields only (no secrets/keys/raw receipts): mode, latch state, verified/rejected counters, last verification timestamps
- Stable schema for dashboard
- Unit tests validating schema + non-leakage (no secret patterns).