feat: implement video site per README spec

- Express + EJS + express-session stack (auth/navbar ported from minecraft_launcher)
- Public: main folder list, folder video grid, internal popup player (/player/:videoId)
- Admin (/op): login, folder CRUD with right-click context menu + add-folder modal
- Admin folder: video grid with right-click edit/rename/delete, "영상 추가" -> editor
- Video editor: drag-drop upload, file picker, YouTube URL probe (ETA + 5분 경고),
  background yt-dlp download with progress polling, navbar title edit, trim controls,
  save runs ffmpeg trim (original preserved)
- Filesystem storage under data/folders/<name>/<videoId>/{meta.json, original.<ext>, edited.<ext>}

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/auth.ts

@@ -0,0 +1,19 @@
+import type { Request, Response, NextFunction } from 'express'
+
+declare module 'express-session' {
+  interface SessionData {
+    userId?: string
+  }
+}
+
+export function requireAuth(req: Request, res: Response, next: NextFunction): void {
+  if (req.session?.userId) {
+    next()
+    return
+  }
+  if (req.method === 'GET') {
+    res.redirect('/op')
+    return
+  }
+  res.status(401).json({ ok: false, message: '인증이 필요합니다.' })
+}