System database backup

Separate from app backups. Backs up SimpleDeploy’s own state (apps, users, configs, metrics). Lives in internal/store/db_backup.go and internal/api/system.go.

• Uses SQLite VACUUM INTO for atomic, consistent copies (WAL-safe)
• Compact mode strips metrics and request_stats tables before download to reduce size
• Managed from the System page in the UI, not the Backups page

For full disaster-recovery procedures, see Disaster recovery.