Live provider health

Report the current state of each provider: `available` (key configured) or `unavailable` (`reason: missing_api_key`). Cheap to serve — no upstream calls, just env-key presence per deployment. Safe to poll. Circuit-breaker state used to flow through this endpoint (`degraded`, `circuit_open`, `circuit_half_open`); it has been removed from the public surface because breaker state is a per-machine load-balancer implementation detail. Customers should retry transparently on 5xx and read the `X-VoxRouter-Breaker-State` response header if they need that signal. Internal observability flows through Honeycomb span attribute `vr.breaker.state`.

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

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