🩹(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/tests/test_utils.py

@@ -2,13 +2,68 @@
 Test utils functions
 """
 
+# pylint: disable=W0621
 import json
 from unittest import mock
 
+from django.conf import settings
+
+import jwt
 import pytest
 from livekit.api import TwirpError
 
-from core.utils import NotificationError, create_livekit_client, notify_participants
+from core.factories import UserFactory
+from core.utils import (
+    NotificationError,
+    create_livekit_client,
+    generate_token,
+    notify_participants,
+)
+
+pytestmark = pytest.mark.django_db
+
+
+def decode_token(token: str) -> dict:
+    """Decode a LiveKit JWT access token for inspection."""
+    return jwt.decode(
+        token,
+        settings.LIVEKIT_CONFIGURATION["api_secret"],
+        algorithms=["HS256"],
+    )
+
+
+def test_generate_token_authenticated_uses_full_name():
+    """The token's display name should default to the user's full name."""
+    user = UserFactory(full_name="Jane Doe")
+
+    token = generate_token(room="my-room", user=user)
+
+    claims = decode_token(token)
+    assert claims["name"] == "Jane Doe"
+    assert claims["sub"] == str(user.sub)
+
+
+def test_generate_token_authenticated_fallback_user_representation():
+    """
+    When the user has no full name, the token's display name should fall back
+    to the user's string representation.
+    """
+    user = UserFactory(full_name=None)
+
+    token = generate_token(room="my-room", user=user)
+
+    claims = decode_token(token)
+    assert claims["name"] == str(user)
+
+
+def test_generate_token_explicit_username_overrides_default():
+    """An explicitly provided username should take precedence over the full name."""
+    user = UserFactory(full_name="Jane Doe")
+
+    token = generate_token(room="my-room", user=user, username="Custom Name")
+
+    claims = decode_token(token)
+    assert claims["name"] == "Custom Name"
 
 
 @mock.patch("asyncio.get_running_loop")

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)