Fix bind_live_broadcast: stream_id come keyword arg.

La gem google-apis-youtube_v3 richiede stream_id: e non un terzo argomento
posizionale, causando 500 in creazione sessione YouTube.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/youtube/broadcast_service.rb

@@ -38,7 +38,7 @@ module Youtube
         )
       )
       stream_result = client.insert_live_stream("snippet,cdn", stream)
-      client.bind_live_broadcast(result.id, "id,contentDetails", stream_result.id)
+      client.bind_live_broadcast(result.id, "id,contentDetails", stream_id: stream_result.id)
 
       {
         broadcast_id: result.id,