Route all discovery fetches through global rate limiter

- search_youtube, fetch_trending, fetch_featured_channels now use _meta_run
- Replaced ThreadPoolExecutor(4) parallel searches with sequential loop
- Replaced ThreadPoolExecutor(3) parallel featured-channel fetches with sequential
- _fetch_and_index_channel passes polite=True to fetch_channel/video_metadata

Discovery was firing 4+ simultaneous yt-dlp processes, each with cookies,
which is what invalidated the session.

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

backend/services/discovery.py

@@ -1,7 +1,6 @@
 """Discovery engine — search-based crawl, trending, community signal, category clustering."""
 import json
 import random
-from concurrent.futures import ThreadPoolExecutor, as_completed
 from datetime import datetime
 from sqlalchemy.orm import Session
 from sqlalchemy import text
@@ -13,7 +12,7 @@ from . import ytdlp
 def _fetch_and_index_channel(db: Session, channel: Channel):
     """Fetch full metadata + recent videos for a discovered channel."""
     try:
-        result = ytdlp.fetch_channel_metadata(channel.youtube_channel_id, max_videos=10)
+        result = ytdlp.fetch_channel_metadata(channel.youtube_channel_id, max_videos=10, polite=True)
         if not result:
             return
         ch_data = result.get("channel", {})
@@ -33,7 +32,7 @@ def _fetch_and_index_channel(db: Session, channel: Channel):
             if not yt_id:
                 continue
             try:
-                meta = ytdlp.fetch_video_metadata(yt_id)
+                meta = ytdlp.fetch_video_metadata(yt_id, polite=True)
                 if meta and meta.get("published_at"):
                     individual_fetched[yt_id] = meta
             except Exception:
@@ -106,29 +105,25 @@ def _search_and_store(
     """Run YouTube searches for the given queries and add results to discovery."""
     discovered: dict[str, dict] = {}
 
-    def _do_search(query: str) -> list[dict]:
+    for query in queries:
         try:
-            return ytdlp.search_youtube(query, max_results=40)
+            results = ytdlp.search_youtube(query, max_results=40)
         except Exception:
-            return []
-
-    with ThreadPoolExecutor(max_workers=4) as pool:
-        futures = {pool.submit(_do_search, q): q for q in queries}
-        for fut in as_completed(futures):
-            for video in fut.result():
-                ch = video.get("channel", {})
-                yt_id = ch.get("youtube_channel_id")
-                name = (ch.get("name") or "").strip()
-                if yt_id and name and yt_id not in followed_yt_ids:
-                    if yt_id not in discovered:
-                        discovered[yt_id] = {"name": name, "count": 0, "previews": []}
-                    discovered[yt_id]["count"] += 1
-                    previews = discovered[yt_id]["previews"]
-                    if len(previews) < 3 and video.get("thumbnail_url") and video.get("title"):
-                        previews.append({
-                            "thumbnail_url": video["thumbnail_url"],
-                            "title": video["title"],
-                        })
+            results = []
+        for video in results:
+            ch = video.get("channel", {})
+            yt_id = ch.get("youtube_channel_id")
+            name = (ch.get("name") or "").strip()
+            if yt_id and name and yt_id not in followed_yt_ids:
+                if yt_id not in discovered:
+                    discovered[yt_id] = {"name": name, "count": 0, "previews": []}
+                discovered[yt_id]["count"] += 1
+                previews = discovered[yt_id]["previews"]
+                if len(previews) < 3 and video.get("thumbnail_url") and video.get("title"):
+                    previews.append({
+                        "thumbnail_url": video["thumbnail_url"],
+                        "title": video["title"],
+                    })
 
     if not discovered:
         return
@@ -613,18 +608,12 @@ def update_graph_signal(db: Session, user_id: int):
 
     sample = random.sample(list(followed_rows), min(12, len(followed_rows)))
 
-    def _fetch(yt_id: str) -> list[str]:
-        try:
-            return ytdlp.fetch_featured_channels(yt_id)
-        except Exception:
-            return []
-
     featured_map: dict[str, list[str]] = {}
-    with ThreadPoolExecutor(max_workers=3) as pool:
-        futures = {pool.submit(_fetch, row["youtube_channel_id"]): row for row in sample}
-        for fut in as_completed(futures):
-            row = futures[fut]
-            featured_map[row["youtube_channel_id"]] = fut.result()
+    for row in sample:
+        try:
+            featured_map[row["youtube_channel_id"]] = ytdlp.fetch_featured_channels(row["youtube_channel_id"])
+        except Exception:
+            featured_map[row["youtube_channel_id"]] = []
 
     needs_indexing: list[int] = []
     for source_yt_id, channel_ids in featured_map.items():

backend/services/ytdlp.py

@@ -152,7 +152,7 @@ def _normalize_channel(info: dict) -> dict:
 
 def search_youtube(query: str, max_results: int = 40) -> list[dict]:
     """Search YouTube via yt-dlp. Uses --flat-playlist for fast results."""
-    stdout, _, code = _run([
+    stdout, _, code = _meta_run([
         "yt-dlp",
         f"ytsearch{max_results}:{query}",
         "--dump-json",
@@ -199,7 +199,7 @@ def fetch_trending(region: str = "US", max_results: int = 50) -> list[dict]:
     region = region.upper()
     # CAI%3D = sort by upload date; gl= sets the region
     url = f"https://www.youtube.com/results?search_query=trending&sp=CAI%253D&gl={region}"
-    stdout, _, code = _run([
+    stdout, _, code = _meta_run([
         "yt-dlp",
         url,
         "--dump-json",
@@ -474,7 +474,7 @@ def fetch_featured_channels(channel_id: str) -> list[str]:
         url = f"https://www.youtube.com/{channel_id}/channels"
     else:
         url = f"https://www.youtube.com/channel/{channel_id}/channels"
-    stdout, _, code = _run([
+    stdout, _, code = _meta_run([
         "yt-dlp", url,
         "--dump-json",
         "--flat-playlist",