coredns: DoT (:853) + DoH (:443) listeners with self-signed cert

- New Corefile snippet (common) shared across plain DNS / DoT / DoH so
  zone-loading + forward + cache stay DRY across all three transports
- scripts/generate-certs.sh: openssl-only self-signed RSA cert with SANs
  for localhost / 127.0.0.1 / ::1 / coredns / dns.local. Idempotent —
  skips regeneration if cert is valid >24h ahead; FORCE=1 to rotate.
- Key chmod is 0644 so the CoreDNS container's nonroot user can read it
  via the bind mount. Acceptable for local dev; production should mount
  real certs with proper UID/GID.
- DOT_PORT=8853, DOH_PORT=8443 (avoids Caddy already-on-443 collision)
- Makefile: `make certs`, `make test-tls`
- All three transports verified end-to-end (dig +tls, dig +https,
  curl with raw RFC 8484 wire format)

.gitignore

@@ -1,5 +1,8 @@
 # Prepared zones are generated from zones/ by scripts/prepare-zones.sh
 zones-prepared/*.zone
 
+# Self-signed certs (re-generated by scripts/generate-certs.sh)
+certs/*.pem
+
 # Local-only env overrides
 .env.local