fix: universe build event loop 차단 방지

- _PRICE_BATCH 100 → 50 (배치당 부하 감소)
- 배치 완료 후 asyncio.sleep(1) 추가 (event loop yield)

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

app/services/universe_service.py

@@ -30,7 +30,7 @@ from app.services.sec_http_client import SECHttpClient
 logger = logging.getLogger(__name__)
 
 _CHUNK = 3000           # asyncpg 32767-param limit (10 cols × 3000 = 30000)
-_PRICE_BATCH = 100      # tickers per yfinance bulk download
+_PRICE_BATCH = 50       # tickers per yfinance bulk download (smaller = more responsive)
 _SEC_CONCURRENCY = 3    # concurrent SEC EDGAR companyfacts requests
 _SCREEN_PAGE = 250      # max per yf.screen() call
 
@@ -399,6 +399,8 @@ class UniverseService:
                 f"Universe: batch {batch_num}/{total_batches} — "
                 f"{len(batch)} tickers, {len(batch_rows)} snapshot rows"
             )
+            # Yield to event loop between batches to keep API responsive
+            await asyncio.sleep(1)
 
         logger.info(
             f"Universe: build complete — {len(ticker_list)} tickers, "