feat(finalist): add reminderSentAt for confirmation reminders

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

prisma/migrations/20260604140229_add_finalist_reminder_sent_at/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "FinalistConfirmation" ADD COLUMN     "reminderSentAt" TIMESTAMP(3);

prisma/schema.prisma

@@ -2761,6 +2761,7 @@ model FinalistConfirmation {
   declinedAt                  DateTime?
   declineReason               String? // optional free-text on decline
   expiredAt                   DateTime?
+  reminderSentAt              DateTime? // set when the pre-deadline reminder is sent (cron)
   promotedFromWaitlistEntryId String?                    @unique // null for original finalists
   createdAt                   DateTime                   @default(now())
   updatedAt                   DateTime                   @updatedAt