Adopt nanoDNS-style INI config for the guide gateway

Replace the gateway's env-var DNS policy with docker/nanodns.ini, modelled on
https://github.com/drakmor/nanoDNS#config: [general] (redirect_url,
allowed_clients), [upstream] (ordered resolver list + timeout_ms), [overrides]
(mask = IPv4 spoof) and [exceptions] (masks forwarded upstream). Masks use
shell-style wildcards and match case-insensitively; resolution order is
exceptions -> overrides -> NXDOMAIN.

The unmatched-name behaviour is deliberately inverted from upstream nanoDNS:
names matching no rule return NXDOMAIN (default-deny) instead of being forwarded,
so PSN, updates and telemetry stay blocked. The source-client allowlist now lives
in [general] allowed_clients. The INI is baked into the image; CONFIG_FILE can
point at a mounted file for per-host overrides.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

Dockerfile

@@ -25,6 +25,7 @@ COPY app/offsets /srv/public/offsets
 COPY app/payloads /srv/public/payloads
 COPY docker/server.py /srv/server.py
 COPY docker/guide_gateway.py /srv/guide_gateway.py
+COPY docker/nanodns.ini /srv/nanodns.ini
 COPY --from=certificates /certificates /srv/certificates
 
 RUN cd /srv/public && sha256sum -c /tmp/payloads.sha256 \