Fix search latency: bypass rate limiter for user-triggered searches

search_youtube now takes polite=False (default) for instant user
searches and polite=True for background discovery crawls.

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

backend/services/ytdlp.py

@@ -152,9 +152,10 @@ def _normalize_channel(info: dict) -> dict:
     }
 
 
-def search_youtube(query: str, max_results: int = 40) -> list[dict]:
+def search_youtube(query: str, max_results: int = 40, polite: bool = False) -> list[dict]:
     """Search YouTube via yt-dlp. Uses --flat-playlist for fast results."""
-    stdout, _, code = _meta_run([
+    runner = _meta_run if polite else _run
+    stdout, _, code = runner([
         "yt-dlp",
         f"ytsearch{max_results}:{query}",
         "--dump-json",