🩹(backend) fix the LiveKit token to use full name as display name

Use the user's full name as the display name in the emitted LiveKit
token instead of the email.

Requested by several self-hosters.

src/backend/core/utils.py

@@ -109,7 +109,7 @@ def generate_token(
         default_username = "Anonymous"
     else:
         identity = str(user.sub)
-        default_username = str(user)
+        default_username = user.full_name or str(user)
 
     if color is None:
         color = generate_color(identity)