feat: add 5-minute TTL cache for PostgreSQL history queries

src/web_api.py

@@ -5,8 +5,10 @@ FastAPI web interface for water monitoring system
 
 import asyncio
 import os
+import time
 from contextlib import asynccontextmanager
 from datetime import datetime, timedelta
+from threading import Lock
 from typing import Any, Dict, List
 
 import requests
@@ -34,6 +36,11 @@ from .water_scraper_v3 import EnhancedWaterMonitorScraper
 
 logger = get_logger(__name__)
 
+# Simple thread-safe TTL cache for PostgreSQL history queries
+HISTORY_CACHE: Dict[str, tuple] = {}
+HISTORY_CACHE_LOCK = Lock()
+HISTORY_TTL = 300  # 5 minutes
+
 # Dashboard HTML is loaded once at import from src/static/dashboard.html.
 _DASHBOARD_HTML_PATH = os.path.join(os.path.dirname(os.path.abspath(__file__)), "static", "dashboard.html")
 try:
@@ -448,19 +455,28 @@ async def get_postgres_history(
     limit: int = Query(2000, ge=1, le=5000),
 ):
     """Get historical measurements for a station from PostgreSQL."""
+    cache_key = f"{station_code}:{hours}:{limit}"
+    now = time.monotonic()
+    with HISTORY_CACHE_LOCK:
+        cached = HISTORY_CACHE.get(cache_key)
+        if cached and now - cached[0] < HISTORY_TTL:
+            return cached[1]
     try:
         db_config = Config.get_database_config()
         if db_config["type"] != "postgresql":
             raise RuntimeError("PostgreSQL is not configured")
         history = PostgresHistory(db_config["connection_string"])
         end_time = datetime.now()
-        return await asyncio.to_thread(
+        data = await asyncio.to_thread(
             history.station_history,
             station_code,
             end_time - timedelta(hours=hours),
             end_time,
             limit,
         )
+        with HISTORY_CACHE_LOCK:
+            HISTORY_CACHE[cache_key] = (now, data)
+        return data
     except RuntimeError as error:
         raise HTTPException(status_code=503, detail=str(error))
     except Exception as error: