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/routers/channels.py

@@ -80,11 +80,7 @@ def _get_channel_or_404(db: Session, channel_id: int) -> Channel:
 
 
 def _index_channels_batch(channel_ids: list[int], user_id: int, delay: float = 1.5):
-    """Run channel syncs sequentially with a polite delay between requests."""
-    import time
-    for i, cid in enumerate(channel_ids):
-        if i > 0:
-            time.sleep(delay)
+    for cid in channel_ids:
         _index_channel_task(cid, user_id)
 
 
@@ -208,9 +204,7 @@ def _enrich_missing_task(limit: int = 20):
             """),
             {"limit": limit},
         ).mappings().all()
-        for i, row in enumerate(rows):
-            if i > 0:
-                time.sleep(2.0)
+        for row in rows:
             try:
                 meta = ytdlp.fetch_video_metadata(row["youtube_video_id"])
                 if meta:
@@ -824,9 +818,7 @@ def _fetch_popular_task(channel_id: int, youtube_channel_id: str, channel_name:
             _tasks[task_id]["done"] = 0
 
     try:
-        for i, yt_id in enumerate(video_ids):
-            if i > 0:
-                time.sleep(2.0)
+        for yt_id in video_ids:
             try:
                 meta = ytdlp.fetch_video_metadata(yt_id)
                 if meta: