perf: use iOS player client and increase metadata gap to reduce throttling

YouTube has been aggressively throttling the default web client.
The iOS client bypasses PO-token requirements and gets less aggressive
rate limiting from Google's side.

- Add --extractor-args youtube:player_client=ios to all yt-dlp calls:
  fetch_video_metadata, fetch_channel_metadata, search_youtube,
  fetch_available_subs, download_subs_only, fetch_video_comments,
  popular fetch flat-playlist crawl, and start_download
- Increase _META_MIN_GAP from 5s to 12s between metadata calls
- Widen jitter from ±2.5s to ±5s for less predictable request timing

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

backend/routers/channels.py

@@ -743,6 +743,7 @@ 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)
 

backend/services/ytdlp.py

@@ -53,7 +53,7 @@ def _run(args: list[str], timeout: int = 60) -> tuple[str, str, int]:
 # hammering YouTube and invalidating cookies.
 _meta_lock = threading.Lock()
 _meta_last_call: float = 0.0
-_META_MIN_GAP = 5.0   # seconds between any two metadata requests
+_META_MIN_GAP = 12.0  # seconds between any two metadata requests
 
 # Active download counter — _meta_run pauses while any download is running so
 # background discovery and a concurrent download never share the YouTube session
@@ -77,7 +77,7 @@ def _meta_run(args: list[str], timeout: int = 60) -> tuple[str, str, int]:
         now = time.monotonic()
         wait = _META_MIN_GAP - (now - _meta_last_call)
         if wait > 0:
-            time.sleep(wait + random.uniform(0.5, 2.5))
+            time.sleep(wait + random.uniform(1.0, 5.0))
         try:
             return _run(args, timeout=timeout)
         finally:
@@ -211,6 +211,7 @@ def search_youtube(query: str, max_results: int = 40, polite: bool = False) -> l
         "--dump-json",
         "--flat-playlist",
         "--quiet",
+        "--extractor-args", "youtube:player_client=ios",
         *_cookie_args(),
     ], timeout=60)
 
@@ -309,6 +310,7 @@ def fetch_video_metadata(video_id: str, polite: bool = False) -> dict | None:
     base_cmd = [
         "yt-dlp", url,
         "--dump-json", "--no-download", "--no-playlist",
+        "--extractor-args", "youtube:player_client=ios",
     ]
     runner = _meta_run if polite else _run
     stdout, stderr, code = runner([*base_cmd, *cookie_args], timeout=30)
@@ -379,6 +381,7 @@ def fetch_channel_metadata(channel_id: str, max_videos: int = 30, start_video: i
         "--dump-single-json",
         "--flat-playlist",
         "--quiet",
+        "--extractor-args", "youtube:player_client=ios",
         *_cookie_args(),
     ]
     if start_video > 1:
@@ -613,6 +616,7 @@ def download_subs_only(video_id: str, subtitle_langs: str) -> bool:
         "--write-subs", "--write-auto-subs",
         "--sub-langs", subtitle_langs,
         "--convert-subs", "vtt",
+        "--extractor-args", "youtube:player_client=ios",
         "-o", output_template,
         *_cookie_args(),
     ], timeout=60)
@@ -628,7 +632,8 @@ def fetch_available_subs(video_id: str) -> dict:
     BCP-47 lang codes. Manual = human-made; auto = auto-generated captions.
     """
     url = f"https://www.youtube.com/watch?v={video_id}"
-    base_cmd = ["yt-dlp", url, "--dump-json", "--no-download", "--no-playlist"]
+    base_cmd = ["yt-dlp", url, "--dump-json", "--no-download", "--no-playlist",
+                "--extractor-args", "youtube:player_client=ios"]
     cookie_args = _cookie_args()
     stdout, _, code = _meta_run([*base_cmd, *cookie_args], timeout=30)
     if code != 0 and cookie_args:
@@ -670,7 +675,7 @@ def fetch_video_comments(youtube_video_id: str, max_comments: int = 20) -> list[
             "--write-info-json",
             "--write-comments",
             # Format: thread_count,total_count,replies_per_thread,reply_pages
-            "--extractor-args", f"youtube:max_comments={max_comments},{max_comments},0,0;comment_sort=top",
+            "--extractor-args", f"youtube:player_client=ios;max_comments={max_comments},{max_comments},0,0;comment_sort=top",
             "--skip-download",
             "--no-playlist",
             "--output", out_tmpl,
@@ -927,6 +932,7 @@ def start_download(
                     "--no-part", "--no-mtime",
                     "-o", output_template,
                     "--newline", "--progress", "--no-colors",
+                    "--extractor-args", "youtube:player_client=ios",
                     *subtitle_args,
                     *cookie_args,
                 ]