Study: lesson reminders on by default for new accounts; one-time backfill turns them on for existing accounts; v2.46.0

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server/data.js

@@ -1421,6 +1421,7 @@ export function sanitizeStudyUsers(value) {
       remindersPausedUntil: typeof item?.remindersPausedUntil === 'string' ? item.remindersPausedUntil : null,
       reminderFrequency: item?.reminderFrequency === 'weekly' ? 'weekly' : 'immediate',
       weeklyDigestSentAt: typeof item?.weeklyDigestSentAt === 'string' ? item.weeklyDigestSentAt : null,
+      remindersDefaultedAt: typeof item?.remindersDefaultedAt === 'string' ? item.remindersDefaultedAt : null,
     })
   }
   return out.slice(0, MAX_STUDY_USERS)