Tweak incoming activity processing

app/incoming_activities.py

@@ -81,6 +81,7 @@ async def process_next_incoming_activity(db_session: AsyncSession) -> bool:
     next_activity.last_try = now()
 
     try:
+        async with db_session.begin_nested():
             await save_to_inbox(
                 db_session,
                 next_activity.ap_object,