Add study user re-engagement reminder emails; v1.1.19

Sends 7/14/30-day inactivity emails to study users who haven't logged in,
with one-click HMAC-signed unsubscribe and automatic re-arm on next login.

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

server/study-helpers.js

@@ -1,4 +1,4 @@
-import { createHash, randomUUID } from 'node:crypto'
+import { createHash, createHmac, randomUUID } from 'node:crypto'
 import path from 'node:path'
 import { parseCookies, cookieSecureFlag } from './helpers.js'
 import {
@@ -666,3 +666,67 @@ export async function scheduleStudyReminders(sendStudyReminderEmail) {
   state.studyReminders.updatedAt = new Date().toISOString()
   queueStudyRemindersWrite()
 }
+
+// ── Study re-engagement scheduler ─────────────────────────────────────────
+
+const REENGAGEMENT_TIERS = [
+  { key: '7d', days: 7 },
+  { key: '14d', days: 14 },
+  { key: '30d', days: 30 },
+]
+
+function makeUnsubUrl(userId) {
+  const secret = process.env.RESEND_WEBHOOK_TOKEN ?? 'siteforge'
+  const token = createHmac('sha256', secret).update(userId).digest('hex')
+  const canonical = state.cachedSiteContent?.seo?.canonicalUrl || 'https://versebyversewithnate.us/'
+  const base = canonical.endsWith('/') ? canonical.slice(0, -1) : canonical
+  return `${base}/api/study-auth/unsubscribe-reminders?uid=${encodeURIComponent(userId)}&token=${token}`
+}
+
+export async function scheduleReengagementEmails(sendStudyReengagementEmail) {
+  if (!state.cachedSiteContent) return
+  const now = new Date()
+  const nowMs = now.getTime()
+
+  for (const user of state.studyUsers) {
+    if (user.studyRemindersEnabled !== true) continue
+    if (!user.lastLoginAt) continue
+    const lastLogin = new Date(user.lastLoginAt)
+    if (isNaN(lastLogin.getTime())) continue
+    const daysSinceLogin = (nowMs - lastLogin.getTime()) / (1000 * 60 * 60 * 24)
+
+    const sent = typeof user.reengagementSentAt === 'object' && user.reengagementSentAt !== null ? user.reengagementSentAt : {}
+
+    const enrolledStudySlugs = Array.isArray(user.enrolledStudySlugs) ? user.enrolledStudySlugs : []
+    const studies = Array.isArray(state.cachedSiteContent.studies) ? state.cachedSiteContent.studies : []
+    const enrolledStudies = enrolledStudySlugs
+      .map(slug => studies.find(s => normalizeStudySlug(s?.slug) === slug))
+      .filter(Boolean)
+
+    if (enrolledStudies.length === 0) continue
+
+    const firstStudy = enrolledStudies[0]
+    const studyTitle = enrolledStudies.length === 1 ? firstStudy.title : null
+    const canonical = state.cachedSiteContent?.seo?.canonicalUrl || 'https://versebyversewithnate.us/'
+    const base = canonical.endsWith('/') ? canonical.slice(0, -1) : canonical
+    const studyUrl = enrolledStudies.length === 1
+      ? `${base}/study/${firstStudy.slug}`
+      : `${base}/study`
+
+    for (const tier of REENGAGEMENT_TIERS) {
+      if (daysSinceLogin < tier.days) continue
+      if (sent[tier.key]) continue
+
+      const unsubUrl = makeUnsubUrl(user.id)
+      try {
+        await sendStudyReengagementEmail(user.username, user.displayName || user.username, studyTitle, studyUrl, tier.key, unsubUrl)
+      } catch (err) {
+        console.error(`[study-reengagement] failed to send ${tier.key} to ${user.username}:`, err)
+        continue
+      }
+      user.reengagementSentAt = { ...sent, [tier.key]: new Date().toISOString() }
+      queueStudyUsersWrite()
+      break  // one tier per run so we don't spam if they've been gone >30 days
+    }
+  }
+}