Create scripts/migrate-users.ts that:
- Creates a default admin user from DEFAULT_ADMIN_EMAIL/DEFAULT_ADMIN_PASSWORD env vars
- Backfills user_id on all existing rows in charts, annotations, annotation_types,
span_annotations, span_label_types
- Is idempotent (safe to run multiple times)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
