docs(infra): add DNS split-horizon topology

infrastructure/dns.md

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+# DNS
+
+**Last updated:** 2026-08-11
+
+## Split-Horizon DNS
+
+The institute runs split-horizon DNS: internal and external services share
+different domain namespaces to prevent internal service URLs from leaking
+publicly.
+
+| Scope | Domain | Usage |
+|-------|--------|-------|
+| Internal | `pldyn.net` | Services accessible only inside the management subnet |
+| External | `sundialer.net` | Services exposed to the public internet |
+
+## Wildcard Patterns
+
+| Scope | Pattern |
+|-------|---------|
+| Internal | `*.pldyn.net` |
+| External | `*.sundialer.net` |
+
+## Certificate Issuance
+
+- **Provider:** Cloudflare (DNS challenge)
+- **Managed by:** Traefik — cert issuance and renewal are automatic
+- **Challenge method:** DNS-01 (no HTTP challenge port required)
+
+Traefik uses Cloudflare API credentials (stored as Swarm secrets / env vars,
+not in documentation) to complete the DNS challenge and obtain Let's Encrypt
+certificates for both domains.
+
+## Notes
+
+- DNS records for `*.pldyn.net` resolve to internal IPs; they are not
+  publicly routable.
+- DNS records for `*.sundialer.net` resolve to the public-facing IP managed
+  by Unifi / Cloudflare.