[Brev][Policy&Network] Port-80 shareable page intermittently returns 503 no_healthy_upstream

## Description

On Sep 11, 2026, a fresh NemoClaw Brev Launchable instance intermittently returned a port-80 reverse-proxy 503 even though Brev reported the instance as RUNNING, build COMPLETED, shell READY, and health HEALTHY. The page did not reach the NemoClaw onboarding UI during the affected attempt.

Platform scope: Brev Launchable only.
Reproducibility: Observed once. A later fresh instance from the same Launchable did not reproduce the 503.
OpenShell issue: Not reached.

## Environment

```text
Device: Brev Launchable, Free n2d-standard-4 CPU instance
OS: Ubuntu 22.04.5 LTS (same Launchable image; captured on a later companion instance)
Architecture: x86_64 (same Launchable image; captured on a later companion instance)
Node.js: v22.23.2 (same Launchable image; captured on a later companion instance)
npm: 10.9.8 (same Launchable image; captured on a later companion instance)
Docker: 29.1.3 (same Launchable image; captured on a later companion instance)
OpenShell CLI: Not captured on the affected instance; later companion instance reported 0.0.106
NemoClaw: Not captured on the affected instance because port 80 was unavailable; the same Daily Tag Launchable was separately verified as v0.0.122 on a later fresh instance
OpenClaw: N/A (onboarding UI was not reached)
```

## Steps to Reproduce

1. Open the NemoClaw Daily Tag Brev Launchable deployment page.
2. Select the Free entitlement and deploy a fresh instance with the default compute settings.
3. Wait until Brev reports RUNNING, build COMPLETED, shell READY, and health HEALTHY.
4. Open the shareable URL linked to port 80.
5. Observe the HTTP response before starting onboarding.
6. If the page loads normally, repeat with a fresh instance because the symptom is intermittent.

## Expected Result

The port-80 shareable page should load the NemoClaw onboarding UI whenever Brev reports the instance healthy.

## Actual Result

```text
Error 503
Web Server is down
Error Text: no_healthy_upstream
```

## Impact

The user cannot reach the Launchable onboarding UI even though Brev reports the deployment healthy, making the instance appear ready when its exposed application is unavailable.

## Logs

```text
Error 503
Web Server is down
Error Text: no_healthy_upstream
```

Start from the NemoClaw Daily Tag Brev Launchable deployment page and reproduce the port-80 shareable URL failure on fresh instances. Compare Brev's RUNNING, COMPLETED, READY, and HEALTHY states with the exposed upstream and the reported no_healthy_upstream response; done means a healthy instance reliably serves the onboarding UI.

Written by the indexing model from the issue text.