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/review/__tests__/route.test.ts

@@ -3,6 +3,7 @@ import { NextRequest } from 'next/server';
 
 vi.mock('@/lib/db/queries', () => ({
   getReviewCheckpoints: vi.fn(),
+  getNextReviewAt: vi.fn().mockResolvedValue(null),
 }));
 
 import { GET } from '../route';