fix: clear instance history on delete and import

deleteInstance now removes history rows for that vmid before removing
the instance. importInstances clears all history before replacing
instances. Prevents stale history appearing when a vmid is reused.

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

server/db.js

@@ -151,11 +151,13 @@ export function updateInstance(vmid, data) {
 }
 
 export function deleteInstance(vmid) {
-  return db.prepare('DELETE FROM instances WHERE vmid = ?').run(vmid);
+  db.prepare('DELETE FROM instance_history WHERE vmid = ?').run(vmid);
+  db.prepare('DELETE FROM instances WHERE vmid = ?').run(vmid);
 }
 
 export function importInstances(rows) {
   db.exec('BEGIN');
+  db.exec('DELETE FROM instance_history');
   db.exec('DELETE FROM instances');
   const insert = db.prepare(`
     INSERT INTO instances