Add study account engagement features; v1.2.4

- Pause reminders: remindersPausedUntil field + scheduler skip + account UI with 7/14/30-day presets
- Email digest mode: reminderFrequency field (immediate/weekly) + weekly digest email + scheduler + account UI toggle
- Admin user list improvements: streak, reengagement, frequency, pause, per-study lesson progress
- Admin manual re-engagement: POST endpoint + tier selector button in admin panel
- Study bookmarks: bookmarkedSectionIds in progress file + POST/DELETE endpoints + toggle button in lesson UI
- Study completion certificate email: auto-sent on first certificate issuance
- Content fields for certificate and weekly digest email templates in admin

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/routes/study-data.js

@@ -171,6 +171,7 @@ export function register(app) {
 
     const progress = await loadUserProgress(user.id)
     const completedSectionIds = progress.byStudy[studySlug]?.completedSectionIds ?? []
+    const bookmarkedSectionIds = progress.byStudy[studySlug]?.bookmarkedSectionIds ?? []
 
     // Record first visit timestamp on the user record (additive, never overwrite)
     if (!user.firstVisitAt) {
@@ -178,7 +179,7 @@ export function register(app) {
       queueStudyUsersWrite()
     }
 
-    res.json({ studySlug, completedSectionIds })
+    res.json({ studySlug, completedSectionIds, bookmarkedSectionIds })
   })
 
   app.post('/api/study-progress/:studySlug/:sectionId', requireStudyAuth, async (req, res) => {
@@ -301,6 +302,45 @@ export function register(app) {
     res.json({ ok: true, studySlug, sectionId, answers })
   })
 
+  // ── Bookmarks ─────────────────────────────────────────────────────────────
+
+  app.post('/api/study-progress/:studySlug/:sectionId/bookmark', requireStudyAuth, async (req, res) => {
+    const user = req.studyUser
+    const studySlug = normalizeStudySlug(req.params.studySlug)
+    const sectionId = normalizeLessonSectionId(req.params.sectionId)
+    if (!studySlug || !sectionId) { res.status(400).json({ message: 'Invalid study slug or section id.' }); return }
+    if (!isStudyUserEnrolled(user, studySlug)) { res.status(403).json({ message: 'Please enroll in this study to bookmark lessons.' }); return }
+
+    const progress = await loadUserProgress(user.id)
+    const studyProgress = progress.byStudy[studySlug] ?? { completedSectionIds: [], bookmarkedSectionIds: [] }
+    const bookmarkedSectionIds = Array.isArray(studyProgress.bookmarkedSectionIds) ? studyProgress.bookmarkedSectionIds : []
+    if (!bookmarkedSectionIds.includes(sectionId)) {
+      studyProgress.bookmarkedSectionIds = [...bookmarkedSectionIds, sectionId]
+    }
+    progress.byStudy[studySlug] = studyProgress
+    progress.updatedAt = new Date().toISOString()
+    state.studyProgressCache.set(user.id, progress)
+    queueUserProgressWrite(user.id)
+    res.json({ ok: true, studySlug, bookmarkedSectionIds: studyProgress.bookmarkedSectionIds })
+  })
+
+  app.delete('/api/study-progress/:studySlug/:sectionId/bookmark', requireStudyAuth, async (req, res) => {
+    const user = req.studyUser
+    const studySlug = normalizeStudySlug(req.params.studySlug)
+    const sectionId = normalizeLessonSectionId(req.params.sectionId)
+    if (!studySlug || !sectionId) { res.status(400).json({ message: 'Invalid study slug or section id.' }); return }
+    if (!isStudyUserEnrolled(user, studySlug)) { res.status(403).json({ message: 'Not enrolled.' }); return }
+
+    const progress = await loadUserProgress(user.id)
+    const studyProgress = progress.byStudy[studySlug] ?? { completedSectionIds: [], bookmarkedSectionIds: [] }
+    studyProgress.bookmarkedSectionIds = (studyProgress.bookmarkedSectionIds ?? []).filter(id => id !== sectionId)
+    progress.byStudy[studySlug] = studyProgress
+    progress.updatedAt = new Date().toISOString()
+    state.studyProgressCache.set(user.id, progress)
+    queueUserProgressWrite(user.id)
+    res.json({ ok: true, studySlug, bookmarkedSectionIds: studyProgress.bookmarkedSectionIds })
+  })
+
   // ── Community ─────────────────────────────────────────────────────────────
 
   app.get('/api/study-community', requireStudyAuth, async (req, res) => {