[FEATURE]: drizzle-gateway - Support configuring the server listen address for IPv6 private networks

### Feature hasn't been suggested before.

- [x] I have verified this feature I'm about to request hasn't been suggested before.

### Describe the enhancement you want to request

I am running Drizzle Gateway behind Cloudflare Tunnel on Railway, using both official templates:

- [Drizzle Gateway for Railway](https://gateway.drizzle.team/docs/railway)
- [Cloudflare Tunnel for Railway](https://railway.com/deploy/cf-tunnel)

The tunnel connects to:

```text
http://drizzle-gateway.railway.internal:4983
```

However, cloudflared cannot connect:

```text
dial tcp [fd12:...]:4983: connect: connection refused
```

[Railway recommends listening on `::`](https://docs.railway.com/private-networking) for IPv6 private networking.

The published Gateway image (`ghcr.io/drizzle-team/gateway:1.4.0`) appears to bind to `0.0.0.0` and only reads the `PORT` environment variable. Therefore, setting `HOST=::` and redeploying had no effect.

Could Gateway support a `HOST` or `HOSTNAME` environment variable, or listen on `::` by default?

Using a public Railway URL as the tunnel origin is undesirable because it creates another public path that may bypass Cloudflare Access.

The issue identifies the published Gateway image and its current PORT environment variable, but names no source file or test. Start by locating the Gateway server entry point and listen configuration, then check how an address setting should support Railway's IPv6 private networking. Done means Gateway can listen on :: for the tunnel origin without requiring a public URL.

Written by the indexing model from the issue text.