Blue AV Envelope (Q3) no longer asks for a nonprofit authorization code
after confirming nonprofit status - it wasn't needed there. Ballot
Mailing Service (Q4) gains the same follow-up instead, with an "I don't
know my authorization code" checkbox that lets the applicant skip it.

Adds MailingNonprofitAuthCode and MailingNonprofitAuthCodeUnknown
columns to OrderDetails via migration, wired through the model,
repository, answers mapper, and confirmation email by hand rather than
via GenerateRepo.vbs, since regenerating would have dropped existing
fixups (leading-zero-preserving text fields, nullable boolean/date
guards) already baked into those files.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>