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.

frontend/src/components/AlertMap.tsx

@@ -1,10 +1,10 @@
-import { useEffect, useRef, useState, useCallback } from 'react';
+import { useEffect, useRef, useState, useCallback, memo } from 'react';
 import L from 'leaflet';
 import { useRiskStore } from '@/stores';
 import { LodGridLayer } from '@/components/LodGridLayer';
 import { GridStatsOverlay } from '@/components/GridStatsOverlay';
 import { useLodGrid } from '@/hooks/useLodGrid';
-import type { Alert } from '@/types';
+import type { Alert, GridRisk } from '@/types';
 
 export interface CellInfo {
   lat: number;
@@ -44,6 +44,8 @@ function getRiskLabel(value: number): string {
   return '低风险';
 }
 
+const EMPTY_GRIDS: GridRisk[] = [];
+
 function AlertMapComponent({
   selectedGridId,
   onGridClick,
@@ -62,7 +64,7 @@ function AlertMapComponent({
   const clickHandlerRef = useRef(onGridClick);
   const [currentZoom, setCurrentZoom] = useState(10);
 
-  const grids = useRiskStore((s) => s.grids ?? []);
+  const grids = useRiskStore((s) => s.grids ?? EMPTY_GRIDS);
 
   // LOD grid data for stats overlay
   const { count, avgRisk, maxRisk, loading } = useLodGrid(currentZoom, forecastDay);
@@ -299,4 +301,4 @@ function AlertMapComponent({
   );
 }
 
-export const AlertMap = AlertMapComponent;
+export const AlertMap = memo(AlertMapComponent);