✨(backend) add metadata collection of VAD, connection and chat events

Introduce MetadataCollector and MetadataCollectorService classes to
centralize the collection and storage of user connections, VAD events,
and chat messages. This creates a structured foundation for future speaker
assignment logic based on voice activity detection. Add tests for this new
feature.

env.d/development/common.dist

@@ -71,6 +71,9 @@ RECORDING_DOWNLOAD_BASE_URL=http://localhost:3000/recording
 # Telephony
 ROOM_TELEPHONY_ENABLED=True
 
+# Metadata
+METADATA_COLLECTOR_ENABLED=True
+
 FRONTEND_USE_FRENCH_GOV_FOOTER=False
 FRONTEND_USE_PROCONNECT_BUTTON=False