coredns: docker compose stack with Vultr zone import

- Auto plugin loads zones-prepared/*.zone (regex zone-name extraction)
- scripts/prepare-zones.sh transforms raw Vultr exports:
  * synthesizes SOA (omitted by Vultr; CoreDNS requires it)
  * prepends @ to leading-TAB apex lines to disambiguate owner inheritance
  * dot-terminates NS/MX/CNAME rdata so $ORIGIN doesn't double-suffix
- DNS_PORT defaults to 1053 (5353=avahi, 53=libvirt dnsmasq on this host)
- Forwards non-authoritative queries to 1.1.1.1/1.0.0.1/9.9.9.9
- Makefile targets: prep, up, down, reload, test, logs
- 91 zones loaded

Corefile

@@ -0,0 +1,25 @@
+. {
+    # Authoritative: load every <zone>.zone in /zones via the auto plugin.
+    # Filename pattern (.*)\.zone yields the zone name from the first group.
+    # CoreDNS reloads modified files every 30s.
+    auto {
+        directory /zones (.*)\.zone {1}
+        reload 30s
+    }
+
+    # Anything not authoritative falls through to upstream resolvers.
+    forward . 1.1.1.1 1.0.0.1 9.9.9.9 {
+        max_concurrent 1000
+    }
+
+    # In-memory cache (TTL clamp 30s for both pos/neg).
+    cache 30
+
+    # Operational plugins
+    health :8080
+    prometheus :9153
+    errors
+    log
+    loop
+    reload 10s
+}