fix: Tier2 backup status now detects drives removed from storage (not just disconnected)

Previously, removing a storage drive from the controller only marked it as
disconnected if the StoragePath entry still existed with Disconnected:true.
Drives removed entirely from storage_paths were invisible to the check,
causing Tier2 backup UI to show green "Sikeres" and scheduler to attempt
backups to a no-longer-managed destination.

New IsStoragePathKnown() method covers both cases. UI shows yellow
"Cél meghajtó leválasztva" and scheduler skips silently.

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

controller/internal/backup/crossdrive.go

@@ -100,6 +100,10 @@ func (r *CrossDriveRunner) RunAppBackup(ctx context.Context, stackName string) e
 		r.logger.Printf("[WARN] [backup] Cross-drive backup skipped for %s: destination drive disconnected (%s)", stackName, cfg.DestinationPath)
 		return nil
 	}
+	if !r.sett.IsStoragePathKnown(cfg.DestinationPath) {
+		r.logger.Printf("[WARN] [backup] Cross-drive backup skipped for %s: destination not a registered storage (%s)", stackName, cfg.DestinationPath)
+		return nil
+	}
 
 	// Mark as running in settings
 	_ = r.sett.UpdateCrossDriveStatus(stackName, func(c *settings.CrossDriveBackup) {