Add also-notify support for BIND domains.conf generation

Extend catz.yaml config with a bind-conf section mapping catalog names
to also-notify IP lists. Zones in catalogs with also-notify configured
get an also-notify directive in the generated domains.conf. IPs are
deduplicated and sorted when a zone belongs to multiple catalogs.

config.go

@@ -19,10 +19,16 @@ type SOAConfig struct {
 	Rname string `yaml:"rname"`
 }
 
+// BindConfConfig holds BIND-specific configuration for domains.conf generation.
+type BindConfConfig struct {
+	AlsoNotify map[string][]string `yaml:"also-notify"`
+}
+
 // Config holds the parsed catz.yaml configuration.
 type Config struct {
 	Catalogs map[string]CatalogConfig `yaml:"catalogs"`
 	SOA      SOAConfig                `yaml:"soa"`
+	BindConf BindConfConfig           `yaml:"bind-conf"`
 }
 
 func loadConfig(path string) (*Config, error) {
@@ -66,6 +72,13 @@ func validateConfig(cfg *Config) error {
 	if cfg.SOA.Rname == "" {
 		return fmt.Errorf("config: soa.rname is required")
 	}
+
+	for catName := range cfg.BindConf.AlsoNotify {
+		if _, ok := cfg.Catalogs[catName]; !ok {
+			return fmt.Errorf("config: bind-conf.also-notify references unknown catalog %q", catName)
+		}
+	}
+
 	return nil
 }