Avoid heavy weight function object

Also, by switching to template we can avoid the cost of converting to some specific type and
perfectly forward the parameter to the final function.

PR #21572.

src/webui/api/torrentscontroller.cpp

@@ -28,6 +28,7 @@
 
 #include "torrentscontroller.h"
 
+#include <concepts>
 #include <functional>
 
 #include <QBitArray>
@@ -135,7 +136,9 @@ namespace
 
     const QSet<QString> SUPPORTED_WEB_SEED_SCHEMES {u"http"_s, u"https"_s, u"ftp"_s};
 
-    void applyToTorrents(const QStringList &idList, const std::function<void (BitTorrent::Torrent *torrent)> &func)
+    template <typename Func>
+    void applyToTorrents(const QStringList &idList, Func func)
+        requires std::invocable<Func, BitTorrent::Torrent *>
     {
         if ((idList.size() == 1) && (idList[0] == u"all"))
         {