feat(stock): apply webai_cache to portfolio/news/screener-preview (SP-A2)

3 endpoint cache 적용 — /api/webai/portfolio, /api/webai/news-sentiment,
/api/stock/screener/run (preview 모드만, auto는 캐시 미적용).
V1+V2 동시 호출도 NAS에서 1회 계산. web-ai 측 SP-A1 캐시와 2-layer로
작동하여 NAS 인바운드 부담 70% 감소 예상.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

stock/app/conftest.py

@@ -0,0 +1,15 @@
+"""Project-level pytest conftest.
+
+SP-A2: autouse fixture that resets all webai_cache TTLCaches between tests
+so screener/portfolio/news cache state does not leak across test cases.
+"""
+import pytest
+from app import webai_cache
+
+
+@pytest.fixture(autouse=True)
+def _reset_webai_cache():
+    webai_cache.PORTFOLIO_CACHE.clear()
+    webai_cache.NEWS_CACHE.clear()
+    webai_cache.SCREENER_CACHE.clear()
+    yield