Fix MultipleResultsFound crash in get_activity_for_source

- Use .scalars().first() + LIMIT 1 instead of scalar_one_or_none()
  which crashes when multiple activities exist for the same source
- Allow re-Create after Delete (re-publish after unpublish)
- Add missing on_post_unpublished handler to root shared copy
- Sync add_follower upsert fix to root shared

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/federation_impl.py

@@ -251,8 +251,9 @@ class SqlFederationService:
                     APActivity.source_type == source_type,
                     APActivity.source_id == source_id,
                 ).order_by(APActivity.created_at.desc())
+                .limit(1)
             )
-        ).scalar_one_or_none()
+        ).scalars().first()
         return _activity_to_dto(a) if a else None
 
     # -- Followers ------------------------------------------------------------