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>

next.config.ts

@@ -2,6 +2,7 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   output: "standalone",
+  transpilePackages: ["unified", "remark-parse"],
 };
 
 export default nextConfig;