feat: Cap number of emails to one per day

posthog/tasks/email.py

@@ -171,7 +171,13 @@ async def send_batch_export_run_failure(
         BatchExportRun.objects.select_related("batch_export__team").get
     )(id=batch_export_run_id)
     team: Team = batch_export_run.batch_export.team
-    campaign_key: str = f"batch_export_run_email_batch_export_{batch_export_run.batch_export.id}_data_interval_end_{batch_export_run.data_interval_end}"
+    # NOTE: We are taking only the date component to cap the number of emails at one per day per batch export.
+    last_updated_at_date = batch_export_run.last_updated_at.strftime("%Y-%m-%d")
+
+    campaign_key: (
+        str
+    ) = f"batch_export_run_email_batch_export_{batch_export_run.batch_export.id}_last_updated_at_{last_updated_at_date}"
+
     message = await sync_to_async(EmailMessage)(
         campaign_key=campaign_key,
         subject=f"PostHog: {batch_export_run.batch_export.name} batch export run failure",