Fix comment fetching: write to temp file instead of parsing stdout

--write-comments writes to .info.json reliably; parsing stdout with
--dump-json was never guaranteed to include comments. Use a TemporaryDirectory,
write the info.json there, read it, then let the context manager clean up.

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

backend/services/ytdlp.py

@@ -386,45 +386,36 @@ def fetch_channel_links(channel_id: str) -> list[str]:
 
 
 def fetch_video_comments(youtube_video_id: str, max_comments: int = 20) -> list[dict]:
-    """Fetch top comments via yt-dlp Python API. Returns empty list on failure."""
-    import yt_dlp as _yt_dlp
+    """Fetch top comments via yt-dlp CLI writing to a temp file. Returns empty list on failure."""
+    import os
+    import tempfile
 
-    with _cookies_lock:
-        cf = _cookies_file
-        b = _cookies_browser
-        oauth2 = _use_oauth2
+    url = f"https://www.youtube.com/watch?v={youtube_video_id}"
 
-    ydl_opts = {
-        "quiet": True,
-        "no_warnings": True,
-        "skip_download": True,
-        "getcomments": True,
-        "extractor_args": {
-            "youtube": {
-                "max_comments": [str(max_comments), str(max_comments)],
-                "comment_sort": ["top"],
-            }
-        },
-    }
-    if oauth2:
-        ydl_opts["username"] = "oauth2"
-        ydl_opts["password"] = ""
-    elif cf and Path(cf).exists():
-        ydl_opts["cookiefile"] = cf
-    else:
-        for candidate in _AUTO_COOKIES_PATHS:
-            if Path(candidate).exists():
-                ydl_opts["cookiefile"] = candidate
-                break
-        else:
-            if b and not cf:
-                ydl_opts["cookiesfrombrowser"] = (b,)
+    with tempfile.TemporaryDirectory() as tmpdir:
+        out_tmpl = os.path.join(tmpdir, "%(id)s.%(ext)s")
+        args = [
+            "yt-dlp", url,
+            "--write-info-json",
+            "--write-comments",
+            "--extractor-args", f"youtube:max_comments={max_comments};comment_sort=top",
+            "--no-download",
+            "--no-playlist",
+            "--quiet",
+            "--output", out_tmpl,
+            *_cookie_args(),
+        ]
+        _run(args, timeout=90)
 
+        info = None
+        for fname in os.listdir(tmpdir):
+            if fname.endswith(".info.json"):
                 try:
-        with _yt_dlp.YoutubeDL(ydl_opts) as ydl:
-            info = ydl.extract_info(f"https://www.youtube.com/watch?v={youtube_video_id}", download=False)
+                    with open(os.path.join(tmpdir, fname)) as f:
+                        info = json.load(f)
                 except Exception:
-        return []
+                    pass
+                break
 
     if not info:
         return []