🔧(backend) set explicit user subscription permissions for other tracks

Configure LiveKit token to explicitly allow users to subscribe to other
participants' video and audio tracks instead of relying on default
permissions.

src/backend/core/utils.py

@@ -90,6 +90,7 @@ def generate_token(
         can_update_own_metadata=True,
         can_publish=bool(sources),
         can_publish_sources=sources,
+        can_subscribe=True,
     )
 
     if user.is_anonymous: