feat: implement 8 features across full stack

- Leaderboard: rank-based percentile tiers (champion/gold/silver/bronze),
    deterministic tie-breaking (xp DESC, user_id ASC)
  - Mentor chat: md: breakpoints, safe-area-inset-bottom padding
  - Learning progress: server-side progressPct, curriculum expansion safety
  - Physical orders: hard-disabled with 410 Gone
  - Certificate approval: pending/issued flow, admin issue/revoke endpoints,
    certificate_requires_approval setting (migration 0031)
  - Draft/publish: is_published on lessons and chapters, admin publish UI
    (migration 0030)
  - Video panel: static thumbnails (no preload), client-side pagination
  - Rich editor: TipTap + tiptap-markdown for lesson descriptions,
    marked + DOMPurify for SafeMarkdown rendering

frontend/package.json

@@ -20,13 +20,19 @@
     "@radix-ui/react-switch": "^1.1.2",
     "@radix-ui/react-tabs": "^1.1.2",
     "@tanstack/react-query": "^5.62.0",
+    "@tiptap/extension-placeholder": "^3.32.0",
+    "@tiptap/react": "^3.32.0",
+    "@tiptap/starter-kit": "^3.32.0",
+    "@types/dompurify": "^3.0.5",
     "boring-avatars": "^2.0.4",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "date-fns": "^4.1.0",
+    "dompurify": "^3.4.17",
     "emoji-datasource-apple": "^15.1.2",
     "framer-motion": "^12.0.0",
     "lucide-react": "^0.468.0",
+    "marked": "^18.1.1",
     "next": "15.5.27",
     "next-themes": "^0.4.4",
     "react": "^19.0.0",
@@ -34,6 +40,7 @@
     "react-multi-date-picker": "^4.5.2",
     "sonner": "^1.7.1",
     "tailwind-merge": "^2.6.0",
+    "tiptap-markdown": "^0.9.0",
     "tw-animate-css": "^1.4.0",
     "zustand": "^5.0.2"
   },