Add global yt-dlp metadata rate limiter (5s + jitter between calls)

All fetch_video_metadata / fetch_channel_metadata / fetch_channel_playlists
/ fetch_available_subs calls now go through _meta_run which enforces a
minimum 5s gap (+ 0.5-2.5s random jitter) across all concurrent tasks.
Per-task sleep loops removed since the global lock serializes everything.

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

backend/services/ytdlp.py

@@ -1,8 +1,10 @@
 """Subprocess wrapper for yt-dlp."""
 import json
+import random
 import re
 import subprocess
 import threading
+import time
 import urllib.request
 import xml.etree.ElementTree as ET
 from datetime import datetime, timezone
@@ -17,6 +19,24 @@ def _run(args: list[str], timeout: int = 60) -> tuple[str, str, int]:
     return result.stdout, result.stderr, result.returncode
 
 
+# Global rate limiter for all metadata fetches — prevents concurrent tasks from
+# 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
+
+
+def _meta_run(args: list[str], timeout: int = 60) -> tuple[str, str, int]:
+    global _meta_last_call
+    with _meta_lock:
+        now = time.monotonic()
+        wait = _META_MIN_GAP - (now - _meta_last_call)
+        if wait > 0:
+            time.sleep(wait + random.uniform(0.5, 2.5))
+        _meta_last_call = time.monotonic()
+    return _run(args, timeout=timeout)
+
+
 def _parse_date(date_str: str | None) -> datetime | None:
     if not date_str:
         return None
@@ -233,13 +253,13 @@ def fetch_video_metadata(video_id: str) -> dict | None:
         "yt-dlp", url,
         "--dump-json", "--no-download", "--no-playlist",
     ]
-    stdout, stderr, code = _run([*base_cmd, *cookie_args], timeout=30)
+    stdout, stderr, code = _meta_run([*base_cmd, *cookie_args], timeout=30)
     if code != 0:
         print(f"[fetch_meta] FAILED code={code} stderr={stderr[:500]!r}", flush=True)
         # Retry without auth args — broken cookie config shouldn't block public videos
         if cookie_args:
             print(f"[fetch_meta] retrying without cookie args", flush=True)
-            stdout, stderr, code = _run(base_cmd, timeout=30)
+            stdout, stderr, code = _meta_run(base_cmd, timeout=30)
             if code != 0:
                 print(f"[fetch_meta] retry also FAILED code={code}", flush=True)
 
@@ -310,7 +330,7 @@ def fetch_channel_metadata(channel_id: str, max_videos: int = 30, start_video: i
         end = (start_video - 1 + max_videos) if start_video > 1 else max_videos
         args += ["--playlist-end", str(end)]
 
-    stdout, _, code = _run(args, timeout=60)
+    stdout, _, code = _meta_run(args, timeout=60)
     if not stdout.strip():
         return None
 
@@ -359,7 +379,7 @@ def fetch_channel_playlists(channel_id: str, max_results: int = 100) -> list[dic
         url = f"https://www.youtube.com/{channel_id}/playlists"
     else:
         url = f"https://www.youtube.com/channel/{channel_id}/playlists"
-    stdout, _, code = _run([
+    stdout, _, code = _meta_run([
         "yt-dlp", url,
         "--dump-json", "--flat-playlist",
         "--playlist-end", str(max_results),
@@ -552,9 +572,9 @@ def fetch_available_subs(video_id: str) -> dict:
     url = f"https://www.youtube.com/watch?v={video_id}"
     base_cmd = ["yt-dlp", url, "--dump-json", "--no-download", "--no-playlist"]
     cookie_args = _cookie_args()
-    stdout, _, code = _run([*base_cmd, *cookie_args], timeout=30)
+    stdout, _, code = _meta_run([*base_cmd, *cookie_args], timeout=30)
     if code != 0 and cookie_args:
-        stdout, _, code = _run(base_cmd, timeout=30)
+        stdout, _, code = _meta_run(base_cmd, timeout=30)
 
     for line in stdout.splitlines():
         line = line.strip()