fix: replace unbounded caches with TTLCache to prevent memory leaks

- fetcher._realtime_cache: dict → TTLCache(maxsize=50, ttl=30)
- news_service._news_cache: dict → TTLCache(maxsize=50, ttl=1800)
- LIGHTWEIGHT_MODE already exists (200→50 trees), just needs env var on Render

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

data/fetcher.py

@@ -277,7 +277,8 @@ def get_stock_info(stock_no: str) -> dict:
 # non-trading-hours requests (twstock price=None) return stale but valid data.
 # ---------------------------------------------------------------------------
 
-_realtime_cache: dict[str, dict] = {}
+from cachetools import TTLCache
+_realtime_cache: TTLCache = TTLCache(maxsize=50, ttl=30)
 
 
 def get_realtime_quote(stock_no: str) -> dict:

services/news_service.py

@@ -18,9 +18,9 @@ logger = logging.getLogger(__name__)
 GEMINI_API_KEY = os.environ.get("GEMINI_API_KEY", "")
 _GEMINI_URL = "https://generativelanguage.googleapis.com/v1beta/models/gemini-2.0-flash:generateContent"
 
-# In-memory cache: {stock_no: (result, timestamp)}
-_news_cache: dict = {}
-_NEWS_TTL = 1800  # 30 minutes
+# In-memory cache with TTL and size limit
+from cachetools import TTLCache
+_news_cache: TTLCache = TTLCache(maxsize=50, ttl=1800)
 
 
 def get_stock_news(stock_no: str, name: str = "", lang: str = "zh-TW") -> Optional[dict]:
@@ -39,12 +39,11 @@ def get_stock_news(stock_no: str, name: str = "", lang: str = "zh-TW") -> Option
     if not GEMINI_API_KEY:
         return None
 
-    # Check cache
+    # Check cache (TTLCache handles expiry automatically)
     now = time.time()
-    if stock_no in _news_cache:
-        cached, ts = _news_cache[stock_no]
-        if now - ts < _NEWS_TTL:
-            return {**cached, "cached": True}
+    cached = _news_cache.get(stock_no)
+    if cached is not None:
+        return {**cached, "cached": True}
 
     stock_label = f"{name} ({stock_no})" if name and name != stock_no else stock_no
 
@@ -95,7 +94,7 @@ def get_stock_news(stock_no: str, name: str = "", lang: str = "zh-TW") -> Option
             "summary": summary,
             "source": "gemini",
         }
-        _news_cache[stock_no] = (result, now)
+        _news_cache[stock_no] = result
         return {**result, "cached": False}
 
     except Exception as exc:

tests/test_arch_deploy.py

@@ -227,7 +227,8 @@ class TestRealtimeQuoteCache:
 
     def test_realtime_cache_exists(self):
         from data.fetcher import _realtime_cache
-        assert isinstance(_realtime_cache, dict)
+        from collections.abc import MutableMapping
+        assert isinstance(_realtime_cache, MutableMapping)
 
     def test_cache_and_return_stores_data(self):
         """get_realtime_quote should store results in _realtime_cache."""