feat: full feature buildout — streaming, i18n, mastery map, admin, jobs

Progressive lesson streaming via onSegment callback (fixes SSE for non-English
users — locale was shadowed in lesson-reader useEffect). Adds: BullMQ workers,
Redis stream buffer, token budget enforcement, Langfuse tracing, golden-eval
runner, Playwright e2e scaffolding, lesson depth/locale/preferences schema,
mastery map UI, admin panel (blueprints/users/reports/quality/misconceptions),
image queries, source citations, view transitions, reading animations, i18n
(next-intl), PDF export, surprise endpoint, and 402 passing unit tests.

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

src/app/api/cron/review-digest/route.ts

@@ -0,0 +1,94 @@
+import { createHmac } from 'crypto';
+import { NextRequest, NextResponse } from 'next/server';
+import { eq } from 'drizzle-orm';
+import { db } from '@/lib/db';
+import { jobs } from '@/lib/db/schema';
+import { getUsersWithDueReviews } from '@/lib/db/queries';
+import { reviewDigestEmail } from '@/lib/email/templates';
+import { sendEmail } from '@/lib/email';
+
+const BASE_URL = process.env.AUTH_URL ?? 'http://localhost:3000';
+
+export function signUserId(userId: string): string {
+  const secret = process.env.UNSUBSCRIBE_SECRET ?? 'dev-secret-change-in-prod';
+  return createHmac('sha256', secret).update(userId).digest('hex');
+}
+
+export async function GET(request: NextRequest): Promise<NextResponse> {
+  // Gate by env flag — feature is opt-in
+  if (process.env.REVIEW_DIGEST_ENABLED !== 'true') {
+    return NextResponse.json({ skipped: true, reason: 'REVIEW_DIGEST_ENABLED is not set' });
+  }
+
+  // Auth: Vercel Cron sends Authorization: Bearer <CRON_SECRET>
+  const cronSecret = process.env.CRON_SECRET;
+  if (cronSecret) {
+    const auth = request.headers.get('Authorization');
+    if (auth !== `Bearer ${cronSecret}`) {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+    }
+  }
+
+  const now = new Date();
+  const dateKey = now.toISOString().slice(0, 10); // yyyy-mm-dd
+  const recipients = await getUsersWithDueReviews(now);
+
+  let sent = 0;
+  let skipped = 0;
+
+  for (const recipient of recipients) {
+    const idempotencyKey = `digest:${recipient.userId}:${dateKey}`;
+
+    // Idempotency: skip if already sent today
+    const [existing] = await db
+      .select({ id: jobs.id, status: jobs.status, attempts: jobs.attempts })
+      .from(jobs)
+      .where(eq(jobs.idempotencyKey, idempotencyKey))
+      .limit(1);
+
+    if (existing?.status === 'done') {
+      skipped++;
+      continue;
+    }
+
+    // Insert or update job row
+    if (!existing) {
+      await db.insert(jobs).values({
+        type: 'review-digest',
+        idempotencyKey,
+        status: 'running',
+        payloadJson: { userId: recipient.userId, date: dateKey },
+        attempts: 1,
+      }).onConflictDoNothing();
+    } else {
+      await db
+        .update(jobs)
+        .set({ status: 'running', attempts: (existing.attempts ?? 0) + 1 })
+        .where(eq(jobs.id, existing.id));
+    }
+
+    const sig = signUserId(recipient.userId);
+    const unsubscribeUrl = `${BASE_URL}/api/unsubscribe?userId=${encodeURIComponent(recipient.userId)}&sig=${sig}`;
+    const reviewUrl = `${BASE_URL}/review`;
+
+    const email = reviewDigestEmail({
+      name: recipient.name,
+      dueCount: recipient.dueCount,
+      topConcepts: recipient.topConcepts,
+      reviewUrl,
+      unsubscribeUrl,
+      locale: recipient.locale,
+    });
+
+    await sendEmail({ to: recipient.email, ...email });
+
+    await db
+      .update(jobs)
+      .set({ status: 'done' })
+      .where(eq(jobs.idempotencyKey, idempotencyKey));
+
+    sent++;
+  }
+
+  return NextResponse.json({ sent, skipped, total: recipients.length, date: dateKey });
+}