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/serial.go

@@ -101,8 +101,8 @@ func (e *Engine) guardSerials(ctx context.Context, base, work *model.State, bump
 			continue
 		}
 		old := base.Zones[z.ID]
-		if old == nil || old.Type != model.ZonePrimary || old.ViewID != z.ViewID {
-			continue // new to named: nothing loaded to compare with
+		if old == nil || old.Type != model.ZonePrimary || old.ViewID != z.ViewID || old.Disabled || z.Disabled {
+			continue // new to named (or not in it): nothing loaded to compare with
 		}
 		path := render.ZoneFilePath(work, z, e.cfg.Paths)
 		if cur, ok, _ := readFile(path); ok && string(cur) == render.ZoneFile(work, z) {