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

stock/app/webai_cache.py

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+"""SP-A2 — NAS stock의 /api/webai/* 엔드포인트 in-memory TTLCache.
+
+web-ai 측 캐시(stock_client._TTL)가 miss됐을 때도 NAS에서 같은 데이터를
+KIS·LLM 재호출 없이 즉시 반환하기 위한 2-layer 캐시의 server 측.
+V1+V2가 동시 호출해도 NAS는 1회만 계산.
+
+TTL 정책 (spec §10 SP-A2):
+- portfolio:  120s (web-ai TTL 180s 보다 짧게 — 변경 감지 가능)
+- news:       600s (sentiment는 일 단위)
+- screener:   180s
+"""
+from __future__ import annotations
+
+import hashlib
+import json
+from typing import Any, Optional
+
+from cachetools import TTLCache
+
+
+PORTFOLIO_CACHE: TTLCache = TTLCache(maxsize=1, ttl=120.0)
+NEWS_CACHE: TTLCache = TTLCache(maxsize=10, ttl=600.0)
+SCREENER_CACHE: TTLCache = TTLCache(maxsize=10, ttl=180.0)
+
+
+# ----- portfolio -----
+
+def cache_get_portfolio() -> Optional[Any]:
+    return PORTFOLIO_CACHE.get("result")
+
+
+def cache_set_portfolio(value: Any) -> None:
+    PORTFOLIO_CACHE["result"] = value
+
+
+# ----- news-sentiment -----
+
+def _news_key(date: Optional[str]) -> str:
+    return date if date else "latest"
+
+
+def cache_get_news(date: Optional[str]) -> Optional[Any]:
+    return NEWS_CACHE.get(_news_key(date))
+
+
+def cache_set_news(date: Optional[str], value: Any) -> None:
+    NEWS_CACHE[_news_key(date)] = value
+
+
+# ----- screener -----
+
+def _screener_key(mode: str, top_n: int, weights: Optional[dict]) -> str:
+    """mode + top_n + weights canonical hash. weights 객체 동등성을 키로."""
+    if weights is None:
+        w_repr = "none"
+    else:
+        # canonical: sorted keys → md5 hex (긴 weights도 짧은 키로)
+        canon = json.dumps(weights, sort_keys=True, ensure_ascii=False)
+        w_repr = hashlib.md5(canon.encode("utf-8")).hexdigest()[:12]
+    return f"{mode}:{top_n}:{w_repr}"
+
+
+def cache_get_screener(mode: str, top_n: int, weights: Optional[dict]) -> Optional[Any]:
+    return SCREENER_CACHE.get(_screener_key(mode, top_n, weights))
+
+
+def cache_set_screener(mode: str, top_n: int, weights: Optional[dict], value: Any) -> None:
+    SCREENER_CACHE[_screener_key(mode, top_n, weights)] = value