Refactor retry logic to reduce code duplication

- Extract retry condition check into ShouldRetryRequest() helper
- Extract backoff delay calculation into CalculateBackoffDelay() helper
- Use helper methods in both GetRecentTracksRequestFinished and GetTopTracksRequestFinished
- Improves code maintainability and consistency

Co-authored-by: jonaski <10343810+jonaski@users.noreply.github.com>

src/scrobbler/lastfmimport.h

@@ -80,6 +80,9 @@ class LastFMImport : public JsonBaseRequest {
   void SendGetRecentTracksRequest(GetRecentTracksRequest request);
   void SendGetTopTracksRequest(GetTopTracksRequest request);
 
+  bool ShouldRetryRequest(const JsonObjectResult &result) const;
+  int CalculateBackoffDelay(const int retry_count) const;
+
   void Error(const QString &error, const QVariant &debug = QVariant()) override;
 
   void UpdateTotalCheck();