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/ops.go

@@ -240,6 +240,9 @@ func setZoneFields(st *model.State, z *model.Zone, in *ZoneInput, creating bool)
 	if in.SerialPolicy != nil {
 		z.SerialPolicy = strings.ToLower(strings.TrimSpace(*in.SerialPolicy))
 	}
+	if in.Disabled != nil {
+		z.Disabled = *in.Disabled
+	}
 	if in.DNSSECPolicy != nil {
 		p := strings.Trim(strings.TrimSpace(*in.DNSSECPolicy), `"`)
 		switch strings.ToLower(p) {