fix: add Navidrome UserAgent in all outgoing requests (#6020)

* There has been a report about navidrome hitting listenbrainz hard
and the listenbrainz guys wanting to be able to distinguish navidrome

* feat: apply Navidrome User-Agent to all outgoing HTTP requests

Add utils/httpclient, a shared http.Client factory whose transport sets
the User-Agent header (Navidrome/{version} - https://github.com/navidrome)
on any request that does not already have one, and use it at every place
the server builds an HTTP client: Last.fm, ListenBrainz and Deezer agents
and auth routers, insights collector, backgrounds handler, and the plugin
host HTTP service. Plugin-set User-Agent values are preserved. The
per-request header lines from the previous commit are superseded by the
transport.

---------

Co-authored-by: Deluan <deluan@navidrome.org>

plugins/host_httpclient.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/navidrome/navidrome/log"
 	"github.com/navidrome/navidrome/plugins/host"
+	"github.com/navidrome/navidrome/utils/httpclient"
 )
 
 const (
@@ -46,7 +47,7 @@ func newHTTPService(pluginName string, permission *HTTPPermission) *httpServiceI
 		requiredHosts: requiredHosts,
 	}
 	svc.client = &http.Client{
-		Transport: http.DefaultTransport,
+		Transport: httpclient.NewTransport(nil),
 		// Timeout is set per-request via context deadline, not here.
 		// CheckRedirect validates hosts and enforces redirect limits.
 		CheckRedirect: func(req *http.Request, via []*http.Request) error {