prepare-db creates pg_statistic in wrong schema + needs Supabase support

Problem

1. config/target-db/init.sql creates public.pg_statistic but verify checks for postgres_ai.pg_statistic
2. prepare-db doesn't support managed Postgres services like Supabase (role creation and ALTER USER fail)

Expected

1. All config files should use postgres_ai.pg_statistic consistently
2. prepare-db --provider supabase should skip steps that don't work on Supabase