♻️(backend) refactor LiveKit access token handling

Switched to using query parameters instead of GET requests, enabling the
inclusion of additional parameters when calling the create endpoint via POST.

Simplified the access token generation process by removing redundant calls to
`with_identity` and consolidating token generation steps. This streamlines the
method flow by preparing all necessary data before generating the access token.

src/backend/core/api/serializers.py

@@ -123,7 +123,7 @@ class RoomSerializer(serializers.ModelSerializer):
         if role is not None or instance.is_public:
             slug = f"{instance.id!s}".replace("-", "")
 
-            username = request.GET.get("username", None)
+            username = request.query_params.get("username", None)
 
             output["livekit"] = {
                 "url": settings.LIVEKIT_CONFIGURATION["url"],