revert: remove ios player_client arg from all yt-dlp calls

It caused timeouts and failures on search/flat-playlist/metadata
operations — those extractors don't support the ios client path.
Keeping only the _META_MIN_GAP increase (5s → 12s) which only
affects background polite calls.

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

backend/routers/channels.py

@@ -743,7 +743,6 @@ def _fetch_popular_task(channel_id: int, youtube_channel_id: str, channel_name:
         "yt-dlp", url,
         "--dump-json", "--flat-playlist",
         "--quiet",
-        "--extractor-args", "youtube:player_client=ios",
         *ytdlp._cookie_args(),
     ], timeout=120)