🚑️(stats) fix tracking id in upload event

Previously, upload event tracking used the user's email instead of
their user ID, preventing reliable association between uploads
and users.

src/backend/chat/views.py

@@ -425,7 +425,7 @@ class ChatConversationAttachmentViewSet(
         if settings.POSTHOG_KEY:
             posthog.capture(
                 "item_uploaded",
-                distinct_id=request.user.email,
+                distinct_id=request.user.id,
                 properties={
                     "id": attachment.pk,
                     "file_name": attachment.file_name,