Only one writeable campaign version at a time

server/app/api/v1/campaigns.py

@@ -288,11 +288,11 @@ def fork_version_for_edit(
     session: Session = Depends(get_session),
     principal: ApiPrincipal = Depends(require_scope("campaign:write")),
 ):
-    """Create a new editable campaign version from a locked/validated/sent version.
+    """Create the campaign's next and only editable working version.
 
-    Versions that were validated, built, queued or sent are immutable audit
-    snapshots. This endpoint makes an explicit editable copy and makes that
-    new copy the campaign's current version.
+    A new working copy may be created only after the current version is
+    permanently user-locked or delivery-final. Validation and temporary user
+    locks must be removed in place instead of creating parallel drafts.
     """
 
     payload = payload or CampaignVersionUpdateRequest()
@@ -324,6 +324,8 @@ def fork_version_for_edit(
             campaign=CampaignResponse.model_validate(campaign),
             version=CampaignVersionResponse.model_validate(version),
         )
+    except LockedCampaignVersionError as exc:
+        raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail=str(exc)) from exc
     except CampaignPersistenceError as exc:
         raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=str(exc)) from exc