🔒️(backend) bind accepted lobby entries to the current username

Bind an accepted lobby entry to the username the participant had at
the moment of acceptance.

This prevents a participant, once accepted, from changing their
display name and reusing the same lobby grant to enter the room
under a different identity.

src/backend/core/services/lobby.py

@@ -150,7 +150,7 @@ class LobbyService:
             livekit_config = utils.generate_livekit_config(
                 room_id=room_id,
                 user=user,
-                username=username,
+                username=participant.username,
                 color=participant.color,
                 configuration=room.configuration,
                 participant_id=participant.id,
@@ -171,7 +171,7 @@ class LobbyService:
             livekit_config = utils.generate_livekit_config(
                 room_id=room_id,
                 user=user,
-                username=username,
+                username=participant.username,
                 color=participant.color,
                 configuration=room.configuration,
                 participant_id=participant.id,

src/backend/core/tests/services/test_lobby.py

@@ -378,6 +378,42 @@ def test_request_entry_accepted_participant(
     )
 
 
+@mock.patch("core.utils.generate_livekit_config")
+def test_request_entry_accepted_participant_username_is_bound(
+    mock_generate_config, lobby_service, participant_id, settings
+):
+    """An accepted identifier must join under the username the host accepted.
+
+    The participant identifier is a bearer value: a stolen or replayed
+    identifier must not be able to enter the room under a different
+    display name than the one the acceptance decision was made on.
+    """
+    settings.LOBBY_KEY_PREFIX = "mocked-cache-prefix"
+    user = AnonymousUser()
+
+    room = RoomFactory(access_level=RoomAccessLevel.RESTRICTED)
+
+    cache.set(
+        f"mocked-cache-prefix_{room.id}_{participant_id}",
+        {
+            "id": participant_id,
+            "username": "accepted-name",
+            "status": "accepted",
+            "color": "#123456",
+        },
+    )
+
+    mock_generate_config.return_value = {"token": "test-token"}
+
+    participant, livekit_config = lobby_service.request_entry(
+        room, user, "spoofed-name", participant_id=participant_id
+    )
+
+    assert participant.status == LobbyParticipantStatus.ACCEPTED
+    assert livekit_config == {"token": "test-token"}
+    assert mock_generate_config.call_args.kwargs["username"] == "accepted-name"
+
+
 @mock.patch("core.utils.generate_livekit_config")
 def test_request_entry_participant_with_role(
     mock_generate_config, lobby_service, participant_id, username, settings