feat(monitor): show backup tunnel latencies

[Sagit-chu]

Summary

• probe latency and packet loss for alternate entry, relay, and exit candidates
• show alternate tunnel paths in the monitoring detail view while preserving compatibility with historical quality data
• include chain details in the latest-quality database fallback query
• add regression coverage for candidate latency reporting and persisted chain details

Validation

• cd go-backend && go test ./...
• cd vite-frontend && pnpm run lint
• cd vite-frontend && pnpm run build
• browser QA: Monitor → Tunnels → tunnel detail displays alternate candidate paths

Closes #508