Attest Deletion

Records the business's own attestation that this category's data was deleted -- the actual "Deletion Proof." This is a self-reported fact, not something OpenProof independently verifies, same evidentiary honesty as every other self-reported field in the product. collection_started_at/retention_days are left untouched -- the row stays a complete historical record of what was collected, for how long, and when deletion was attested.