✨(backend) add minimal Recording viewset for room recordings

Implements routes to manage recordings within rooms, following the patterns
established in Impress. The viewset exposes targeted endpoints rather than
full CRUD operations, with recordings being created (soon) through
room-specific routes (e.g. room/123/start-recording).

The implementation draws from @sampaccoud's initial work and advices.

Review focus areas:
- Permission implementation choices
- Serializer design and structure

Credit: Initial work by @sampaccoud

src/backend/core/urls.py

@@ -12,6 +12,7 @@ from core.authentication.urls import urlpatterns as oidc_urls
 router = DefaultRouter()
 router.register("users", viewsets.UserViewSet, basename="users")
 router.register("rooms", viewsets.RoomViewSet, basename="rooms")
+router.register("recordings", viewsets.RecordingViewSet, basename="recordings")
 router.register(
     "resource-accesses", viewsets.ResourceAccessViewSet, basename="resource_accesses"
 )