✨(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/frontend/src/features/rooms/utils/getParticipantIsAuthenticated.ts

@@ -0,0 +1,8 @@
+import type { Participant } from 'livekit-client'
+
+export const getParticipantIsAuthenticated = (
+  participant: Participant
+): boolean => {
+  const isAuthenticated = participant.attributes?.is_authenticated
+  return isAuthenticated == 'true'
+}