Implement IPayee1099Service and Payee1099Service: list/get/create/update/
soft-delete and RevealTin. TIN is encrypted via ITinProtector on write;
TinLast4 is the only clear-text fragment stored. Null Tin on update
preserves the existing ciphertext. Four xUnit tests cover encrypt-on-create,
null-tin-keeps-ciphertext, list-masks-to-last4, and soft-delete hides from list.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Chris Chen