feat: add NSD backend and Topology C (multi-instance with peer sync) 🏗️

- New NSDBackend: zone files + nsd-control reload, zone registration via
  nsd.conf.d include file; mirrors BIND backend interface exactly
- BackendRegistry now supports type "nsd"; config defaults for nsd.zones_dir
  and nsd.nsd_conf
- Dockerfile installs both NSD and BIND9 — entrypoint detects configured
  backend type(s) and starts only the required daemon; CoreDNS MySQL
  deployments start neither
- docker/nsd.conf: minimal NSD base config with remote-control and
  zones.conf include
- entrypoint.sh: reads config file + env vars to determine which daemon
  to start; runs nsd-control-setup on first boot
- 20 new NSD backend tests (117 total, all passing)
- README: Topology C (multi-instance + peer sync) documented as most robust
  HA option; NSD config reference; updated topology comparison table;
  NSD env-var-only compose examples; version 2.5.0

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "directdnsonly"
-version = "2.4.0"
+version = "2.5.0"
 description = "DNS Management System - DirectAdmin to multiple backends"
 authors = [
     {name = "Aaron Guise",email = "aaron@guise.net.nz"}