fix: copy cookies to temp file so yt-dlp doesn't overwrite originals

yt-dlp saves rotated cookies back on exit, destroying fresh exports.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/download.py

@@ -2,7 +2,9 @@
 
 import os
 import re
+import shutil
 import logging
+import tempfile
 from dataclasses import dataclass
 from typing import Optional
 
@@ -38,7 +40,11 @@ def extract_video_id(url: str) -> Optional[str]:
 def _base_opts() -> dict:
     opts = {"quiet": True, "no_warnings": True}
     if COOKIES_FILE and os.path.exists(COOKIES_FILE):
-        opts["cookiefile"] = COOKIES_FILE
+        # Copy cookies to a temp file so yt-dlp doesn't overwrite the original
+        tmp = tempfile.NamedTemporaryFile(delete=False, suffix=".txt")
+        shutil.copy2(COOKIES_FILE, tmp.name)
+        tmp.close()
+        opts["cookiefile"] = tmp.name
     return opts