Improve "last activity" calculation in WebAPI (#15339)

Co-authored-by: 秦风 <mayli.he@gmail.com>

Co-authored-by: Chocobo1 <Chocobo1@users.noreply.github.com>

src/webui/api/serialize/serialize_torrent.cpp

@@ -97,6 +97,14 @@ QVariantMap serialize(const BitTorrent::Torrent &torrent)
         return (ratio > BitTorrent::Torrent::MAX_RATIO) ? -1 : ratio;
     };
 
+    const auto getLastActivityTime = [&torrent]() -> qlonglong
+    {
+        const qlonglong timeSinceActivity = torrent.timeSinceActivity();
+        return (timeSinceActivity < 0)
+            ? torrent.addedTime().toSecsSinceEpoch()
+            : (QDateTime::currentDateTime().toSecsSinceEpoch() - timeSinceActivity);
+    };
+
     return {
         {KEY_TORRENT_ID, torrent.id().toString()},
         {KEY_TORRENT_INFOHASHV1, torrent.infoHash().v1().toString()},
@@ -145,7 +153,7 @@ QVariantMap serialize(const BitTorrent::Torrent &torrent)
         {KEY_TORRENT_AUTO_TORRENT_MANAGEMENT, torrent.isAutoTMMEnabled()},
         {KEY_TORRENT_TIME_ACTIVE, torrent.activeTime()},
         {KEY_TORRENT_SEEDING_TIME, torrent.seedingTime()},
-        {KEY_TORRENT_LAST_ACTIVITY_TIME, (QDateTime::currentDateTime().toSecsSinceEpoch() - torrent.timeSinceActivity())},
+        {KEY_TORRENT_LAST_ACTIVITY_TIME, getLastActivityTime()},
         {KEY_TORRENT_AVAILABILITY, torrent.distributedCopies()},
 
         {KEY_TORRENT_TOTAL_SIZE, torrent.totalSize()}