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/routes/study-comments.js

@@ -1,6 +1,6 @@
 import { randomUUID } from 'node:crypto'
 import { requireAdminAuth } from '../auth.js'
-import { requireStudyAuth, normalizeStudySlug, normalizeLessonSectionId, isStudyUserEnrolled, findStudyUserById, getCommunityDisplayName } from '../study-helpers.js'
+import { requireStudyAuth, normalizeStudySlug, normalizeLessonSectionId, hasStudyAccess, findStudyUserById, getCommunityDisplayName } from '../study-helpers.js'
 import { state } from '../state.js'
 import { queueStudyCommentsWrite } from '../data.js'
 import { MAX_STUDY_COMMENTS, MAX_STUDY_COMMENT_LENGTH } from '../config.js'
@@ -40,7 +40,7 @@ export function register(app) {
       res.status(400).json({ message: 'Invalid study slug or section id.' }); return
     }
 
-    if (!isStudyUserEnrolled(user, studySlug)) {
+    if (!hasStudyAccess(user, studySlug)) {
       res.status(403).json({ message: 'Please enroll in this study to leave a comment.' }); return
     }