Live provider health

Report the current state of each provider: whether it is `available`, `degraded` (half-open circuit-breaker probe in flight), or `unavailable` (circuit-breaker open, or API key not configured on this deployment). Safe to poll — cheap to serve (no upstream calls, just in-memory breaker state + env-key presence). Breaker state is per-process in-memory. Multi-replica deployments expose each replica's independent view; a client polling through a load balancer may observe flapping, which is a true signal of "some replicas see this provider as failing."

curl https://api.voxrouter.ai/v1/status \
  -H "Authorization: Bearer $VOXROUTER_API_KEY"

{
  "providers": [
    {
      "id": "string",
      "state": "available",
      "reason": "missing_api_key"
    }
  ]
}