Make migration policy creation idempotent

DROP POLICY IF EXISTS before CREATE POLICY so migrations can be
re-applied to a DB that already has the policy (e.g. demo_config
was manually applied before migration tracking was set up).

migrations/20260511000000_user_api_keys.sql

@@ -15,4 +15,5 @@ CREATE TABLE IF NOT EXISTS public.user_api_keys (
 );
 
 ALTER TABLE public.user_api_keys ENABLE ROW LEVEL SECURITY;
+DROP POLICY IF EXISTS "service only" ON public.user_api_keys;
 CREATE POLICY "service only" ON public.user_api_keys USING (false);