fix: screener yf.screen() rate limit/401 retry 처리 추가

429/rate limit 및 401/unauthorized 발생 시 최대 3회 재시도 (delay 3s, 6s).
모든 retry 소진 후 RuntimeError로 변환하여 503 응답.

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

app/services/screener_service.py

@@ -153,7 +153,34 @@ class ScreenerService:
     def _screen_sync(self, query, offset: int, size: int, sort_field: str, sort_asc: bool) -> dict:
         """Synchronous yfinance screen() call — must run in executor."""
         import yfinance as yf
+
+        max_retries = 3
+        for attempt in range(max_retries):
+            try:
                 return yf.screen(query, offset=offset, size=size, sortField=sort_field, sortAsc=sort_asc)
+            except Exception as e:
+                err = str(e).lower()
+                is_retriable = (
+                    "too many requests" in err
+                    or "rate limit" in err
+                    or "429" in err
+                    or "401" in err
+                    or "unauthorized" in err
+                )
+                if is_retriable and attempt < max_retries - 1:
+                    delay = (attempt + 1) * 3  # 3s, 6s
+                    logger.warning(
+                        "yfinance screen() error '%s' (attempt %d/%d), retrying in %.1fs",
+                        str(e)[:80], attempt + 1, max_retries, delay,
+                    )
+                    time.sleep(delay)
+                    continue
+                if is_retriable:
+                    raise RuntimeError(
+                        "Yahoo Finance is rate limiting this server. "
+                        "Please try again in 30–60 seconds."
+                    )
+                raise
 
     async def screen_stocks(
         self,