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/data.js

@@ -1489,6 +1489,9 @@ export function sanitizeStudyUsers(value) {
     const enrolledStudySlugs = Array.isArray(item?.enrolledStudySlugs)
       ? Array.from(new Set(item.enrolledStudySlugs.map(normalizeStudySlug).filter(Boolean))).slice(0, MAX_STUDY_ENROLLMENTS_PER_USER)
       : []
+    const completedStudySlugs = Array.isArray(item?.completedStudySlugs)
+      ? Array.from(new Set(item.completedStudySlugs.map(normalizeStudySlug).filter(Boolean))).slice(0, MAX_STUDY_ENROLLMENTS_PER_USER * 4)
+      : []
     const displayName = typeof item?.displayName === 'string' ? item.displayName.trim().slice(0, 80) : ''
     const subscribeNewsletter = item?.subscribeNewsletter !== false
     const studyRemindersEnabled = item?.studyRemindersEnabled === true
@@ -1523,6 +1526,11 @@ export function sanitizeStudyUsers(value) {
       subscribeNewsletter,
       studyRemindersEnabled,
       enrolledStudySlugs,
+      completedStudySlugs,
+      currentStreak: Number.isFinite(item?.currentStreak) && item.currentStreak > 0 ? Math.floor(item.currentStreak) : 0,
+      longestStreak: Number.isFinite(item?.longestStreak) && item.longestStreak > 0 ? Math.floor(item.longestStreak) : 0,
+      lastStudiedDate: typeof item?.lastStudiedDate === 'string' ? item.lastStudiedDate : null,
+      firstCompletionAt: typeof item?.firstCompletionAt === 'string' ? item.firstCompletionAt : null,
       avatarUrl: typeof item?.avatarUrl === 'string' ? item.avatarUrl.trim() : '',
       createdAt: typeof item?.createdAt === 'string' ? item.createdAt : null,
       updatedAt: typeof item?.updatedAt === 'string' ? item.updatedAt : null,