Only one writeable campaign version at a time

server/app/mailer/sending/jobs.py

@@ -175,6 +175,10 @@ def _get_current_version(session: Session, campaign: Campaign, version_id: str |
     version = session.get(CampaignVersion, wanted)
     if not version or version.campaign_id != campaign.id:
         raise QueueingError(f"Campaign version not found or not part of campaign: {wanted}")
+    if campaign.current_version_id != version.id:
+        raise QueueingError(
+            "Historical campaign versions are review-only. Open the current working version before queueing or sending."
+        )
     return version