Completed studies: finishing a study keeps access after leaving; certificates always shown; no reminders for finished studies; v2.11.0

- Completion is stored on the user (completedStudySlugs, sticky) when every lesson is done; backfilled at startup
- Lessons, notes, progress, quiz, community and certificate stay open for completed studies after leaving the enrollment
- Dashboard/My Account list completed studies with Review Lessons and certificate access
- Study page shows a completed banner
- Lesson-release reminders, weekly digest and re-engagement emails skip finished studies
- Fix: streak fields (current/longest/last studied) and firstCompletionAt were dropped on every restart when users were loaded
- Tests for completion/access/re-engagement

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

server.js

@@ -43,6 +43,7 @@ import {
   scheduleStudyReminders,
   scheduleReengagementEmails,
   scheduleWeeklyDigestEmails,
+  backfillCompletedStudies,
 } from './server/study-helpers.js'
 import { recordVisitor } from './server/routes/analytics.js'
 
@@ -195,6 +196,11 @@ Promise.all([
   .catch(err => {
     console.error('[stats] failed to load persisted stats:', err)
   })
+  .then(() => backfillCompletedStudies())
+  .then(updated => { if (updated > 0) console.log(`[study] marked ${updated} finished studies as completed`) })
+  .catch(err => {
+    console.error('[study] completed-study backfill failed:', err)
+  })
   .finally(() => {
     createBackupSnapshot('startup').catch(() => {})
     setInterval(() => {