feat(final-docs): add FinalistConfirmation.finalDocsReminderSentAt

Additive nullable column for the auto document-reminder cron (fires once per
team). Migration hand-authored + applied to dev via db execute (NOT migrate dev,
per drifted-dev-history rule); prod applies it via migrate deploy on next build.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

prisma/schema.prisma

@@ -2762,6 +2762,7 @@ model FinalistConfirmation {
   declineReason               String? // optional free-text on decline
   expiredAt                   DateTime?
   reminderSentAt              DateTime? // set when the pre-deadline reminder is sent (cron)
+  finalDocsReminderSentAt     DateTime? // set when the grand-final document-upload reminder is sent (cron)
   promotedFromWaitlistEntryId String?                    @unique // null for original finalists
   createdAt                   DateTime                   @default(now())
   updatedAt                   DateTime                   @updatedAt