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>

server/db.js

@@ -155,7 +155,7 @@ export function deleteInstance(vmid) {
   db.prepare('DELETE FROM instances WHERE vmid = ?').run(vmid);
 }
 
-export function importInstances(rows) {
+export function importInstances(rows, historyRows = []) {
   db.exec('BEGIN');
   db.exec('DELETE FROM instance_history');
   db.exec('DELETE FROM instances');
@@ -168,6 +168,12 @@ export function importInstances(rows) {
        @tailscale, @andromeda, @tailscale_ip, @hardware_acceleration)
   `);
   for (const row of rows) insert.run(row);
+  if (historyRows.length) {
+    const insertHist = db.prepare(
+      `INSERT INTO instance_history (vmid, field, old_value, new_value, changed_at) VALUES (?, ?, ?, ?, ?)`
+    );
+    for (const h of historyRows) insertHist.run(h.vmid, h.field, h.old_value ?? null, h.new_value ?? null, h.changed_at);
+  }
   db.exec('COMMIT');
 }
 
@@ -177,6 +183,10 @@ export function getInstanceHistory(vmid) {
   ).all(vmid);
 }
 
+export function getAllHistory() {
+  return db.prepare('SELECT * FROM instance_history ORDER BY vmid, changed_at').all();
+}
+
 // ── Test helpers ──────────────────────────────────────────────────────────────
 
 export function _resetForTest() {