🐛(auth) fix LiveKit token authentication field mismatch

Fixes "Invalid LiveKit token" errors caused by field mismatch between
token generation and authentication lookup.

Previously:
- generate_token() used user.sub as token identity
- LiveKitTokenAuthentication tried to retrieve user via user.id field
- This failed when sub was not a UUID (e.g., from LemonLDAP OIDC provider)

Now:
- generate_token() continues using user.sub (canonical OIDC identifier)
- LiveKitTokenAuthentication correctly looks up by sub field
- Both sides now consistently use the same field

This ensures compatibility with all RFC 7519-compliant OIDC providers,
regardless of their sub claim format.

src/backend/core/authentication/livekit.py

@@ -30,7 +30,7 @@ class LiveKitTokenAuthentication(authentication.BaseAuthentication):
                 raise exceptions.AuthenticationFailed("Token missing user identity")
 
             try:
-                user = UserModel.objects.get(id=user_id)
+                user = UserModel.objects.get(sub=user_id)
             except UserModel.DoesNotExist:
                 user = AnonymousUser()