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.js

@@ -31,7 +31,7 @@ import {
   refreshContentCaches,
   queueHitStatsWrite,
 } from './server/data.js'
-import { logResendEmailAlignmentWarnings, sendStudyReminderEmail, sendStudyReengagementEmail } from './server/email.js'
+import { logResendEmailAlignmentWarnings, sendStudyReminderEmail, sendStudyReengagementEmail, sendStudyWeeklyDigestEmail } from './server/email.js'
 import {
   detectBot,
   sanitizeUserAgent,
@@ -39,6 +39,7 @@ import {
   recordHit,
   scheduleStudyReminders,
   scheduleReengagementEmails,
+  scheduleWeeklyDigestEmails,
 } from './server/study-helpers.js'
 import { recordVisitor } from './server/routes/analytics.js'
 
@@ -194,6 +195,14 @@ Promise.all([
     }, 60 * 60 * 1000)
     void scheduleReengagementEmails(sendStudyReengagementEmail)
 
+    // Send weekly lesson digest emails every hour (only actually fires every 7 days per user)
+    setInterval(() => {
+      scheduleWeeklyDigestEmails(sendStudyWeeklyDigestEmail).catch(err => {
+        console.error('[study-digest] failed to schedule weekly digest:', err)
+      })
+    }, 60 * 60 * 1000)
+    void scheduleWeeklyDigestEmails(sendStudyWeeklyDigestEmail)
+
     app.listen(PORT, () => {
       logResendEmailAlignmentWarnings()
       console.log(`Siteforge listening on http://localhost:${PORT}`)