✨(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.

src/backend/core/api/viewsets.py

@@ -45,6 +45,10 @@ from core.recording.event.exceptions import (
 )
 from core.recording.event.notification import notification_service
 from core.recording.event.parsers import get_parser
+from core.recording.services.metadata_collector import (
+    MetadataCollectorException,
+    MetadataCollectorService,
+)
 from core.recording.worker.exceptions import (
     RecordingStartError,
     RecordingStopError,
@@ -338,6 +342,15 @@ class RoomViewSet(
                 status=drf_status.HTTP_500_INTERNAL_SERVER_ERROR,
             )
 
+        if settings.METADATA_COLLECTOR_ENABLED and (
+            recording.mode == models.RecordingModeChoices.TRANSCRIPT
+            or recording.options.get("transcribe", False)
+        ):
+            try:
+                MetadataCollectorService().start(recording)
+            except MetadataCollectorException:
+                logger.warning("Failed to start MetadataCollectorService")
+
         return drf_response.Response(
             {"message": f"Recording successfully started for room {room.slug}"},
             status=drf_status.HTTP_201_CREATED,