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-account.js

@@ -28,6 +28,7 @@ import {
   findStudyUserByUsername,
   getStudyAvatarUrl,
   isStudyUserEnrolled,
+  isStudyUserCompleted,
   normalizeStudySlug,
   getStudyCatalog,
   clearStudySessionCookie,
@@ -212,6 +213,7 @@ export function register(app) {
         title: study.title,
         status: study.status,
         enrolled: isStudyUserEnrolled(user, study.slug),
+        completed: isStudyUserCompleted(user, study.slug),
         totalLessons,
         completedLessons,
         noteCount,