feat: add reports center, admin drill-down, disease filter + bug fixes + perf optimization

Frontend features:
- 报表中心 (ReportsCenter): list/detail views, diagnosis breakdown chart, CSV export
- 多级行政下钻 (AdminBreadcrumb): 湖北省→武汉市→区→街道 hierarchical drill-down
- 按病种筛选 (DiseaseFilter): multi-select diagnosis filter on monitoring + reports pages

Backend:
- Add /forecast/{days} endpoint, diagnosis filter params on cases endpoints
- Add /streets aggregation endpoint, enrich reports with real case data
- Extract shared case_loader module

Bug fixes (14):
- Fix missing /risk/forecast route (404), historyApi pointing to non-existent router
- Fix min_risk filter silently ignored in insights/hotspots
- Fix type mismatches: CaseTrendResponse, CaseStatsResponse shapes
- Fix silent .catch(() => {}) swallowing errors, fetchAlerts not clearing stale state
- Fix lru_cache caching exceptions, generateReport used cachedGet for write op
- Fix missing useEffect deps in Insights, DistrictComparison, ReportsCenter

Performance (9):
- Zustand selectors across 9 components (eliminate re-render cascades)
- Fix districtCases.sort() mutating store state, inline IIFE → memo'd component
- CaseLocationMap: React.memo, race protection, correct deps
- AlertCard: stable callbacks, TimelinePlayer: useMemo, TopNav: clock isolation
- SideNav: modules array to module scope, DiseaseFilter: memoized filter

frontend/src/components/TopNav.tsx

@@ -4,15 +4,16 @@ interface TopNavProps {
   onLogout?: () => void;
 }
 
-export function TopNav({ onLogout }: TopNavProps) {
-  const [currentTime, setCurrentTime] = useState('');
-
+function Clock() {
+  const [time, setTime] = useState(new Date());
   useEffect(() => {
-    const update = () => setCurrentTime(new Date().toLocaleString('zh-CN'));
-    update();
-    const timer = setInterval(update, 1000);
-    return () => clearInterval(timer);
+    const id = setInterval(() => setTime(new Date()), 1000);
+    return () => clearInterval(id);
   }, []);
+  return <span>{time.toLocaleTimeString('zh-CN', { hour: '2-digit', minute: '2-digit' })}</span>;
+}
+
+export function TopNav({ onLogout }: TopNavProps) {
 
   return (
     <nav className="h-[52px] bg-bg-card border-b border-border flex items-center px-5 fixed top-0 left-0 right-0 z-50">
@@ -35,7 +36,7 @@ export function TopNav({ onLogout }: TopNavProps) {
 
       <div className="ml-auto flex items-center gap-5">
         <span className="text-[12px] text-text-muted">
-          {currentTime}
+          <Clock />
         </span>
         <div className="flex items-center gap-2 text-[13px] text-text-secondary">
           <svg className="w-4 h-4" fill="currentColor" viewBox="0 0 24 24">