Disable zones instead of deleting them

A disabled zone stays in Jarl with its records, options and history,
still editable, but named doesn't load it; enabling it serves it as it is
then. Signed zones, and zones another view shares, can't be disabled.
jarl zone disable|enable, the zone page's Disable/Enable button.
Database schema 7.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/expand.go

@@ -145,6 +145,12 @@ func (x *expander) expand(req Request) ([]model.Op, error) {
 			if n := len(x.st.ZoneRecords(z.ID)); n > 0 {
 				verb += fmt.Sprintf(" (with its %d records)", n)
 			}
+		} else if d := zoneToggle(req, z); d != nil {
+			if *d {
+				x.warnings = append(x.warnings, disabledZoneEffect(x.st, z))
+				return []model.Op{x.versioned(req, z.Version, "disable zone "+zoneDesc(x.st, z))}, nil
+			}
+			return []model.Op{x.versioned(req, z.Version, "enable zone "+zoneDesc(x.st, z))}, nil
 		}
 		return []model.Op{x.versioned(req, z.Version, "zone "+verb+" "+zoneDesc(x.st, z))}, nil
 	case OpRecordCreate:
@@ -1029,3 +1035,31 @@ func RedactOps(ops []model.Op) []model.Op {
 	}
 	return out
 }
+
+// zoneToggle returns the new disabled state when a zone update only
+// disables or enables the zone (nil otherwise).
+func zoneToggle(req Request, z *model.Zone) *bool {
+	var in ZoneInput
+	if decode(req.Data, &in) != nil || in.Disabled == nil || *in.Disabled == z.Disabled {
+		return nil
+	}
+	only := ZoneInput{Disabled: in.Disabled}
+	if string(mustJSON(in)) != string(mustJSON(only)) {
+		return nil
+	}
+	return in.Disabled
+}
+
+// disabledZoneEffect says what clients get once named no longer loads a
+// zone: in a view that offers recursion, answers from recursion (the
+// internet's version of the zone, if it has one); elsewhere, REFUSED.
+func disabledZoneEffect(st *model.State, z *model.Zone) string {
+	where := "this server"
+	if v := st.ViewName(z); v != "" {
+		where = "view " + v
+	}
+	if st.RecursionOn(z.ViewID) {
+		return fmt.Sprintf("once zone %s is disabled, clients of %s that may use recursion get its names from recursion: the internet's version of the zone, if there is one", dnsname.Display(z.Name), where)
+	}
+	return fmt.Sprintf("once zone %s is disabled, %s answers REFUSED for its names", dnsname.Display(z.Name), where)
+}