fix: persist config across restarts

- root cause: Technitium image ships /etc/dns as a real directory, so the
  build-time 'ln -sf /data/dns /etc/dns' created the symlink INSIDE it
  (/etc/dns/dns) instead of replacing it. Technitium kept writing to the
  ephemeral container directory and lost everything on restart.
- fix: add entrypoint.sh that runs at container start (after HA mounts
  volumes), removes the real /etc/dns dir, and symlinks it to /config/dns
  (the persistent addon_config mount). Seeds defaults on first run only.
- bump version to 15.4.2

technitium_dns/config.yaml

@@ -1,5 +1,5 @@
 name: Technitium DNS
-version: 15.4.0
+version: 15.4.2
 slug: technitium_dns
 description: Technitium DNS Server add-on for Home Assistant
 arch: