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>

package.json

@@ -5,6 +5,7 @@
   "type": "module",
   "main": "dist/app.js",
   "scripts": {
+    "setup": "node scripts/setup.mjs",
     "build": "tsc -p tsconfig.json",
     "start": "node dist/app.js",
     "dev": "tsc -p tsconfig.json && node dist/app.js"