feat(stock): webai_cache module (TTLCache for SP-A2)

3개의 TTLCache (portfolio 120s · news 600s · screener 180s) +
헬퍼 함수. screener key는 mode + top_n + weights canonical hash로
분기. 다음 커밋에서 /api/webai/portfolio·news-sentiment·screener/run
3 endpoint에 적용.

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

stock/app/test_webai_cache.py

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+"""SP-A2 — webai_cache module의 cache hit/miss + key 분기 검증."""
+import time
+import pytest
+from app.webai_cache import (
+    PORTFOLIO_CACHE, NEWS_CACHE, SCREENER_CACHE,
+    cache_get_portfolio, cache_set_portfolio,
+    cache_get_news, cache_set_news,
+    cache_get_screener, cache_set_screener,
+    _screener_key,
+)
+
+
+def _clear_all():
+    PORTFOLIO_CACHE.clear()
+    NEWS_CACHE.clear()
+    SCREENER_CACHE.clear()
+
+
+def test_portfolio_cache_miss_then_hit():
+    _clear_all()
+    assert cache_get_portfolio() is None
+    cache_set_portfolio({"holdings": [], "cash": 0})
+    assert cache_get_portfolio() == {"holdings": [], "cash": 0}
+
+
+def test_news_cache_key_by_date():
+    """date가 다르면 별도 캐시 슬롯."""
+    _clear_all()
+    cache_set_news("2026-05-18", {"count": 5})
+    cache_set_news("2026-05-17", {"count": 3})
+    assert cache_get_news("2026-05-18") == {"count": 5}
+    assert cache_get_news("2026-05-17") == {"count": 3}
+    assert cache_get_news("2026-05-16") is None  # not cached
+
+
+def test_news_cache_latest_key_normalized():
+    """date=None은 'latest' 키로 정규화되어 동일 슬롯."""
+    _clear_all()
+    cache_set_news(None, {"count": 9})
+    assert cache_get_news(None) == {"count": 9}
+
+
+def test_screener_key_includes_mode_and_top_n():
+    """screener key는 mode + top_n + weights hash로 분기."""
+    k_preview = _screener_key("preview", 20, None)
+    k_preview_w = _screener_key("preview", 20, {"news": 0.3})
+    k_auto = _screener_key("auto", 20, None)
+    assert k_preview != k_preview_w
+    assert k_preview != k_auto
+
+
+def test_screener_cache_roundtrip():
+    _clear_all()
+    payload = {"asof": "2026-05-18", "survivors_count": 17}
+    cache_set_screener("preview", 20, None, payload)
+    assert cache_get_screener("preview", 20, None) == payload
+    assert cache_get_screener("preview", 20, {"news": 0.3}) is None
+
+
+def test_ttl_expiry_portfolio():
+    """짧은 ttl로 만료 확인 — 직접 시간 조작 대신 TTLCache 내부 동작 신뢰."""
+    from cachetools import TTLCache
+    short = TTLCache(maxsize=1, ttl=0.1)  # 0.1초
+    short["result"] = "x"
+    assert short.get("result") == "x"
+    time.sleep(0.2)
+    assert short.get("result") is None