fix(backup): surface real SQLite error when backup step fails

The error-check ordering after backupOp.Step(-1) checked !done before
err, which masked the underlying SQLite error (e.g. SQLITE_BUSY, I/O
errors) with a generic "backup not done with step -1" message. On
failure, Step returns done=false together with a non-nil err, so the
!done branch short-circuited before the real error was ever reported.

Swap the checks so the SQLite error is returned first, making failing
backups actually diagnosable.

Refs https://github.com/navidrome/navidrome/issues/5305#issuecomment-4230470593

db/backup.go

@@ -81,12 +81,12 @@ func backupOrRestore(ctx context.Context, isBackup bool, path string) error {
 			// Caution: -1 means that sqlite will hold a read lock until the operation finishes
 			// This will lock out other writes that could happen at the same time
 			done, err := backupOp.Step(-1)
-			if !done {
-				return fmt.Errorf("backup not done with step -1")
-			}
 			if err != nil {
 				return fmt.Errorf("error during backup step: %w", err)
 			}
+			if !done {
+				return fmt.Errorf("backup not done with step -1")
+			}
 
 			err = backupOp.Finish()
 			if err != nil {