♻️(backend) control metadata collector agent launch via feature flag

Allow controlling when the metadata collector agent is started,
enabling users to try the feature and disable it if needed.

Introduce a user-level feature flag to toggle the agent for the
initial release.

src/backend/core/services/livekit_events.py

@@ -178,9 +178,7 @@ class LiveKitEventsService:
         except utils.MetadataUpdateException as e:
             logger.exception("Failed to update room's metadata: %s", e)
 
-        if settings.METADATA_COLLECTOR_ENABLED and recording.options.get(
-            "metadata_collector_dispatch_id"
-        ):
+        if recording.options.get("metadata_collector_dispatch_id", None) is not None:
             try:
                 MetadataCollectorService().stop(recording)
             except MetadataCollectorException: