📈(backend) capture an event when a thread is created

When a new thread is created, we capture an event related to this thread
creation.

src/backend/core/api/viewsets.py

@@ -3136,12 +3136,21 @@ class ThreadViewSet(
         del serializer.validated_data["body"]
         thread = serializer.save()
 
+        user = self.request.user if self.request.user.is_authenticated else None
+
         models.Comment.objects.create(
             thread=thread,
-            user=self.request.user if self.request.user.is_authenticated else None,
+            user=user,
             body=body,
         )
 
+        posthog_capture(
+            PosthogEventName.THREAD_CREATED,
+            user,
+            {"thread_id": str(thread.id)},
+            document=self.get_document_or_404(),
+        )
+
     @drf.decorators.action(detail=True, methods=["post"], url_path="resolve")
     def resolve(self, request, *args, **kwargs):
         """Resolve a thread."""