feat: include history in export/import backup

Export now returns version 2 with a history array alongside instances.
Import accepts the history array and restores all audit events. v1 backups
without a history key still import cleanly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/db.test.js

@@ -202,6 +202,15 @@ describe('importInstances', () => {
     importInstances([{ ...base, name: 'new', vmid: 2 }]);
     expect(getInstanceHistory(1)).toHaveLength(0);
   });
+
+  it('restores history rows when provided', () => {
+    importInstances(
+      [{ ...base, name: 'a', vmid: 1 }],
+      [{ vmid: 1, field: 'created', old_value: null, new_value: null, changed_at: '2026-01-01 00:00:00' }]
+    );
+    const h = getInstanceHistory(1);
+    expect(h.some(e => e.field === 'created')).toBe(true);
+  });
 });
 
 // ── instance history ─────────────────────────────────────────────────────────