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>

js/ui.js

@@ -384,11 +384,11 @@ async function importDB() {
   document.getElementById('confirm-ok').onclick = async () => {
     closeConfirm();
     try {
-      const { instances } = JSON.parse(await file.text());
+      const { instances, history = [] } = JSON.parse(await file.text());
       const res = await fetch('/api/import', {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({ instances }),
+        body: JSON.stringify({ instances, history }),
       });
       const data = await res.json();
       if (!res.ok) { showToast(data.error ?? 'Import failed', 'error'); return; }