feat: npm run setup (yt-dlp + deps + build); lift upload size limit

scripts/setup.mjs runs `npm install`, downloads the platform-specific
yt-dlp binary from GitHub releases to ./bin/yt-dlp (which src/youtube.ts
already prefers), checks for ffmpeg and prints install hints, then runs
`tsc`. One command replaces three for fresh checkouts.

While verifying setup, hit `MulterError: File too large` (LIMIT_FILE_SIZE)
on a 10 GB mkv upload, and ETXTBSY on freshly downloaded yt-dlp.

- ETXTBSY: the redirect path in downloadFile opened a writestream to the
  destination before following the redirect, so the (unused) outer stream
  still held the file open when the post-download spawnSync ran. Split
  redirect-following from file writing so only the final 200 response
  opens the destination file.
- LIMIT_FILE_SIZE: removed the hard-coded 4 GB cap. Upload limit now
  defaults to Infinity and is configurable via UPLOAD_MAX_BYTES.
  Wrapped multer's middleware so its errors (LIMIT_FILE_SIZE etc.) come
  back as a clean 413 JSON instead of a stack trace from the global
  error handler.
- Also disabled Node's default 5 minute requestTimeout so 10 GB uploads
  over slow links don't get cut mid-stream. Configurable via
  HTTP_REQUEST_TIMEOUT_MS.

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

src/routes/op.ts

@@ -1,6 +1,7 @@
 import { Router } from 'express'
 import path from 'node:path'
-import multer from 'multer'
+import multer, { MulterError } from 'multer'
+import type { Request, Response, NextFunction } from 'express'
 import { promises as fs } from 'node:fs'
 import { requireAuth } from '../auth.js'
 import {
@@ -30,9 +31,13 @@ import { FfmpegUnavailableError, applyTrimToVideo } from '../editor.js'
 
 export const opRouter = Router()
 
+// 업로드 용량 상한은 환경변수 UPLOAD_MAX_BYTES 로 조정. 기본은 사실상 무제한(Infinity).
+const uploadMaxBytes = process.env.UPLOAD_MAX_BYTES
+  ? Math.max(1, Number(process.env.UPLOAD_MAX_BYTES))
+  : Infinity
 const upload = multer({
   dest: tmpDir,
-  limits: { fileSize: 4 * 1024 * 1024 * 1024 } // 4GB
+  limits: { fileSize: uploadMaxBytes }
 })
 
 function pickStr(v: unknown): string {
@@ -181,11 +186,31 @@ opRouter.post('/op/folder/:name/video/delete', requireAuth, async (req, res) =>
   }
 })
 
+// multer 가 던지는 LIMIT_FILE_SIZE 같은 에러를 라우트 핸들러가 잡지 못해
+// 글로벌 에러 핸들러로 새서 stack trace 가 그대로 노출되던 문제를 막는다.
+function uploadSingle(fieldName: string) {
+  const mw = upload.single(fieldName)
+  return (req: Request, res: Response, next: NextFunction) => {
+    mw(req, res, (err: unknown) => {
+      if (!err) return next()
+      if (err instanceof MulterError) {
+        const message =
+          err.code === 'LIMIT_FILE_SIZE'
+            ? `파일이 너무 큽니다. (한도: ${uploadMaxBytes === Infinity ? '무제한' : uploadMaxBytes + ' bytes'}; UPLOAD_MAX_BYTES 환경변수로 조정)`
+            : `업로드 실패: ${err.message}`
+        res.status(413).json({ ok: false, code: err.code, message })
+        return
+      }
+      res.status(400).json({ ok: false, message: (err as Error).message || '업로드 실패' })
+    })
+  }
+}
+
 // 업로드: 단일 파일. multipart/form-data, fields: title, file
 opRouter.post(
   '/op/folder/:name/video/upload',
   requireAuth,
-  upload.single('file'),
+  uploadSingle('file'),
   async (req, res) => {
     try {
       const safe = sanitizeFolderName(req.params.name)