dashboard/app: set coverage batching timeout to 8 hours

Sometimes 6 hours are not enough to make the quarter long aggregation.
Typically 5 hours 30 minutes are enough.

dashboard/app/coverage_batch.go

@@ -27,7 +27,7 @@ func initCoverageBatches() {
 
 const (
 	daysToMerge         = 7
-	batchTimeoutSeconds = 60 * 60 * 6
+	batchTimeoutSeconds = 60 * 60 * 8
 )
 
 func handleBatchCoverage(w http.ResponseWriter, r *http.Request) {