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_test.go

@@ -122,6 +122,36 @@ func TestValidateConfig(t *testing.T) {
 			t.Fatalf("unexpected error: %v", err)
 		}
 	})
+
+	t.Run("also-notify references unknown catalog", func(t *testing.T) {
+		cfg := &Config{
+			Catalogs: map[string]CatalogConfig{"cat1": {Zone: "cat.example.com."}},
+			SOA:      SOAConfig{Mname: "ns1.example.com.", Rname: "hostmaster.example.com."},
+			BindConf: BindConfConfig{
+				AlsoNotify: map[string][]string{
+					"nonexistent": {"198.51.100.1"},
+				},
+			},
+		}
+		if err := validateConfig(cfg); err == nil {
+			t.Fatal("expected error for also-notify referencing unknown catalog")
+		}
+	})
+
+	t.Run("also-notify references valid catalog", func(t *testing.T) {
+		cfg := &Config{
+			Catalogs: map[string]CatalogConfig{"cat1": {Zone: "cat.example.com."}},
+			SOA:      SOAConfig{Mname: "ns1.example.com.", Rname: "hostmaster.example.com."},
+			BindConf: BindConfConfig{
+				AlsoNotify: map[string][]string{
+					"cat1": {"198.51.100.1"},
+				},
+			},
+		}
+		if err := validateConfig(cfg); err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+	})
 }
 
 func TestLoadConfigWritePermission(t *testing.T) {