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>

src/app/api/admin/import-md/route.ts

@@ -0,0 +1,39 @@
+import { NextRequest, NextResponse } from "next/server";
+import { headers } from "next/headers";
+import { auth } from "@/lib/auth";
+import matter from "gray-matter";
+import { mdToTiptap } from "@/lib/md-to-tiptap";
+
+export async function POST(req: NextRequest) {
+  const session = await auth.api.getSession({ headers: await headers() });
+  if (!session || session.user.role !== "admin") {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  const formData = await req.formData();
+  const file = formData.get("file") as File | null;
+
+  if (!file) {
+    return NextResponse.json({ error: "No file provided" }, { status: 400 });
+  }
+  if (!file.name.endsWith(".md")) {
+    return NextResponse.json({ error: "Only .md files are supported" }, { status: 400 });
+  }
+
+  const raw = await file.text();
+  const { data: fm, content } = matter(raw);
+
+  // Extract known frontmatter fields (Obsidian-compatible naming)
+  const title =
+    typeof fm.title === "string" ? fm.title.trim() : null;
+  const kinescopeId =
+    (fm.kinescopeId ?? fm.kinescope_id ?? fm.videoId ?? fm.video_id ?? "") as string;
+  const order =
+    typeof fm.order === "number" ? fm.order : null;
+  const published =
+    typeof fm.published === "boolean" ? fm.published : null;
+
+  const tiptapContent = mdToTiptap(content);
+
+  return NextResponse.json({ title, kinescopeId, order, published, content: tiptapContent });
+}