Add extended forward zone type for overriding public domain records

An extended zone answers authoritatively only for the records it holds
locally; any name with nothing configured falls through to recursive
resolution instead of a synthesized NXDOMAIN, so a public domain's
records don't need to be duplicated just to override a few of them.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V8c76Xr2BycaHHu5N4buqi

internal/web/handlers_zones.go

@@ -174,7 +174,7 @@ func (s *Server) handleZoneDelete(w http.ResponseWriter, r *http.Request) error
 	}
 	setFlash(w, r, "success", fmt.Sprintf("Zone %s and its records were deleted.", strings.TrimSuffix(zone.Name, ".")))
 	target := "/zones"
-	if zone.Kind != models.ZoneForward {
+	if !zone.Kind.IsForward() {
 		target = "/zones/reverse"
 	}
 	return s.redirect(w, r, target)