🐛(backend) fix info panel crash for unregistered rooms

The info panel was crashing when opening a room that was not
registered in the database (with ALLOW_UNREGISTERED_ROOMS=true),
because the API response did not include the room slug.

Instead of adding frontend fallbacks, update the unregistered-room
response to include the slug, keeping the API contract consistent
with registered rooms.

Note: this still relies on the unregistered-room response staying
aligned with the registered-room schema. Any future divergence
between the two responses could introduce similar issues.
A refactoring on the backend side is needed.

It closes #1441.

src/backend/core/api/viewsets.py

@@ -92,6 +92,7 @@ from core.services.subtitle import SubtitleException, SubtitleService
 from core.tasks.file import process_file_deletion
 
 from ..authentication.livekit import LiveKitTokenAuthentication
+from ..models import RoomAccessLevel
 from . import permissions, serializers, throttling
 from .feature_flag import FeatureFlag
 
@@ -267,6 +268,9 @@ class RoomViewSet(
             username = request.query_params.get("username", None)
             data = {
                 "id": None,
+                "slug": slug,
+                "is_administrable": False,
+                "access_level": RoomAccessLevel.PUBLIC,
                 "livekit": {
                     "url": settings.LIVEKIT_CONFIGURATION["url"],
                     "room": slug,