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/discovery.py

@@ -107,7 +107,7 @@ def _search_and_store(
 
     for query in queries:
         try:
-            results = ytdlp.search_youtube(query, max_results=40)
+            results = ytdlp.search_youtube(query, max_results=40, polite=True)
         except Exception:
             results = []
         for video in results: