fix: use NULL for empty phone in create_admin to avoid unique index conflict

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/create_admin.py

@@ -38,7 +38,7 @@ def main() -> None:
                 first_name=args.first_name,
                 last_name=args.last_name,
                 email=args.email,
-                phone=args.phone,
+                phone=args.phone or None,
                 password_hash=hash_password(args.password),
                 role=UserRoleEnum(args.role),
                 status=UserStatusEnum.active,