Anonymizes an applicant's personal data per GDPR right-to-erasure, except for the data/structured_data/secure_data keys listed in data_whitelist, which are preserved as-is. Idempotent — a no-op if the applicant has already been anonymized. A scoped API token needs create and delete access to applicants (applicants: cd). Without create access the request returns 401, and without delete access it returns 403.