Server: Run data importing job using parallel stream

This way, stock data is not fetched sequentially. This reduces fetching
time from ~22 seconds to ~7 seconds on my machine.

server/src/main/java/xyz/laane/server/service/importing/DataImportingJob.java

@@ -91,6 +91,7 @@ public void updateAllStocks() {
         batchRepository.save(batch);
 
         List<Stock> stocks = BALTIC_MAIN_LIST_ISINS.stream()
+            .parallel()
             .map(stockDataImportingService::fetchData)
             .flatMap(Optional::stream)
             .toList();