Database backup and restore

[benSlaughter]

If the server dies, all progress is lost. Parents should be able to backup/restore easily.

Admin settings page:

• "Download Backup" button → downloads spellbound.db file
• "Restore from Backup" → file upload that replaces the database (with confirmation dialog)

API:

• GET /api/admin/backup → streams the SQLite file
• POST /api/admin/restore → accepts multipart upload, validates it's a valid SQLite file, replaces current DB

Safety: Auto-backup before restore (keep last 3 backups in data/backups/).