Add Markdown import from Obsidian (Stage 8)

- md-to-tiptap.ts: remark-based converter (headings, lists, blockquotes,
  code blocks, bold/italic/strike, links, images, hr)
- Obsidian ![[wikilink]] stripped, [[link|alias]] → plain text
- POST /api/admin/import-md: parses frontmatter (gray-matter) + converts content
- LessonEditor: "Импорт .md" button populates editor without auto-save
- ROADMAP: marked Stages 2, 3, 5, 6, 7, 8 as complete, fixed numbering

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

package.json

@@ -33,17 +33,20 @@
     "better-auth": "^1.6.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
+    "gray-matter": "^4.0.3",
     "lucide-react": "^1.7.0",
     "next": "16.2.2",
     "next-themes": "^0.4.6",
     "pg": "^8.20.0",
     "react": "19.2.4",
     "react-dom": "19.2.4",
+    "remark-parse": "^11.0.0",
     "resend": "^6.10.0",
     "shadcn": "^4.1.2",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.5.0",
     "tw-animate-css": "^1.4.0",
+    "unified": "^11.0.5",
     "zod": "^4.3.6"
   },
   "devDependencies": {