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/store/store.go

@@ -300,6 +300,10 @@ CREATE TABLE blocklist_status (
 	sha256 TEXT NOT NULL DEFAULT '',
 	error TEXT NOT NULL DEFAULT ''
 );
+`,
+	// 7: disabled zones, kept but not loaded by named.
+	`
+ALTER TABLE zones ADD COLUMN disabled INTEGER NOT NULL DEFAULT 0;
 `,
 }