✨(backend) expose is_authenticated in the LiveKit token

Include the is_authenticated flag on the user in the LiveKit token
and participant metadata.

The frontend needs this information (used in the next commit) to
know whether it can offer to promote a user with access to the room
admin.

src/backend/core/utils.py

@@ -128,7 +128,11 @@ def generate_token(
         .with_identity(identity)
         .with_name(display_name)
         .with_attributes(
-            {"color": color, "room_role": role }
+            {
+                "color": color,
+                "room_role": role,
+                "is_authenticated": "true" if user.is_authenticated else "false",
+            }
         )
     )