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/services/api.ts

@@ -87,7 +87,7 @@ api.interceptors.response.use(
   }
 );
 
-async function cachedGet<T>(url: string, params?: Record<string, any>): Promise<T> {
+export async function cachedGet<T>(url: string, params?: Record<string, any>): Promise<T> {
   const key = getCacheKey(url, params);
   const cached = getCached<T>(key);
   if (cached !== undefined) return cached;