feat(saas): add down migrations for all incremental schema changes (AUD3-DB-01)

- 16 down SQL files in migrations/down/ for each incremental migration
- db::run_down_migrations() executes rollback files in reverse order
- migrate_down CLI task: task=migrate_down timestamp=20260402
- Initial schema and seed data excluded (would be destructive)

crates/zclaw-saas/migrations/down/20260401000005_rate_limit_events.sql

@@ -0,0 +1,2 @@
+-- Down: Drop rate_limit_events table
+DROP TABLE IF EXISTS rate_limit_events;