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/app/zones.go

@@ -152,7 +152,7 @@ func (a *App) normaliseZoneInput(in ZoneInput, base models.Zone) (models.Zone, s
 		kind = models.ZoneKind(validate.ZoneKindForName(z.Name))
 	}
 	if !kind.Valid() {
-		return z, "", Invalid("Zone kind %q must be forward, reverse4 or reverse6.", in.Kind)
+		return z, "", Invalid("Zone kind %q must be forward, extended, reverse4 or reverse6.", in.Kind)
 	}
 	z.Kind = kind