fix: address review findings for backup management

- HIGH: Add sync.Mutex to backup Engine to prevent concurrent
  backup/restore operations
- HIGH: Restore uses io.Copy instead of ReadFile to avoid OOM on
  large databases
- HIGH: Send HTTP response before closing DB during restore, then
  perform destructive operations in a goroutine
- HIGH: Create pre-restore safety backup before overwriting database
- HIGH: Autobackup cron reschedules dynamically when settings change
  via callback pattern (same as DNS provider changes)

internal/api/settings.go

@@ -205,6 +205,13 @@ func (s *Server) updateSettings(w http.ResponseWriter, r *http.Request) {
 		go s.handleDNSSettingsChange(oldProvider, existing, updated)
 	}
 
+	// Handle backup settings changes.
+	backupChanged := existing.BackupEnabled != updated.BackupEnabled ||
+		existing.BackupIntervalHours != updated.BackupIntervalHours
+	if backupChanged && s.onBackupSettingsChanged != nil {
+		s.onBackupSettingsChanged(updated.BackupEnabled, updated.BackupIntervalHours)
+	}
+
 	respondJSON(w, http.StatusOK, map[string]string{"status": "updated"})
 }