fix: yfinance threads=False + 배치 30개로 OS 스레드 폭발 방지

threads=True일 때 배치당 50개 OS 스레드 생성 → 시스템 자원 고갈 → API 먹통.
threads=False로 순차 다운로드, 배치 30개로 축소.

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 = 50       # tickers per yfinance bulk download (smaller = more responsive)
+_PRICE_BATCH = 30       # tickers per yfinance bulk download
 _SEC_CONCURRENCY = 3    # concurrent SEC EDGAR companyfacts requests
 _SCREEN_PAGE = 250      # max per yf.screen() call
 
@@ -494,7 +494,7 @@ class UniverseService:
                 interval="1mo",
                 auto_adjust=False,
                 progress=False,
-                threads=True,
+                threads=False,   # disable internal thread pool — prevents OS thread explosion
                 group_by="ticker",
             )
         except Exception as e: