fix: analysis 500s, caching, alert page perf

P0: Fix KeyError in 3 analysis endpoints. geojson.py stores 1d risk
as "risk_value" but analysis.py accessed "risk_1d" — always crashed.

Backend: Add lru_cache to GeoJSON/CSV/Parquet loaders, date helpers,
and district loader. Add try/except and FileNotFoundError guards.

Frontend: Debounce riskRange, merge counts into useMemo, stabilize
handleGridClick with ref, memoize nearest-grid scan, wrap AlertMap
in React.memo, switch useLodGrid from fetch to cachedGet.

backend/utils/geojson.py

@@ -2,17 +2,26 @@
 GeoJSON file parsing utilities.
 """
 import json
+import logging
+from functools import lru_cache
 from pathlib import Path
 from typing import Any
 
 from config import WUHAN_BOUNDARY_PATH
 from utils.risk import risk_value_to_level
 
+logger = logging.getLogger(__name__)
 
+
+@lru_cache(maxsize=8)
 def parse_geojson_file(filepath: Path) -> list[dict[str, Any]]:
     """Parse GeoJSON file and extract grid data with standard fields."""
-    with open(filepath, "r", encoding="utf-8") as f:
-        geojson = json.load(f)
+    try:
+        with open(filepath, "r", encoding="utf-8") as f:
+            geojson = json.load(f)
+    except (json.JSONDecodeError, OSError) as e:
+        logger.warning("Failed to parse GeoJSON file %s: %s", filepath, e)
+        return []
 
     grids: list[dict[str, Any]] = []
     for feature in geojson.get("features", []):
@@ -33,6 +42,7 @@ def parse_geojson_file(filepath: Path) -> list[dict[str, Any]]:
     return grids
 
 
+@lru_cache(maxsize=1)
 def load_districts() -> list[dict[str, Any]]:
     """Load Wuhan district boundaries from GeoJSON."""
     if not WUHAN_BOUNDARY_PATH.exists():