feat: P0生产加固 — 认证安全+操作审计+错误提示

P0-1 认证安全:
- JWT secret 改为环境变量配置, 未设置则随机生成(重启失效)
- 新增 LoginRateLimitMiddleware: 5次/分钟登录频率限制(滑动窗口)
- 429 响应 + 前端 toast 提示

P0-2 操作审计日志:
- 新增 OperationLog 模型 (user/username/action/resource/detail/ip)
- 新增 AuditLogMiddleware 自动记录 POST/PUT/DELETE/PATCH 操作
- 新增 GET /audit-logs API (分页/过滤/统计)
- 前端 AuditLogsPage 对接真实 API (字段/路径修正)

P0-3 实例密码加密:
- 已确认 crypto.py AES-256-GCM 加密存储, 无需修改

P1 前端错误提示:
- 新增 useToast hook + ToastContainer 组件
- request.ts 拦截器: 4xx/5xx 错误 toast 显示后端 detail 信息
- 422 验证错误提取第一条 msg
- 网络失败显示通用提示

1182 tests passed

frontend/src/components/ToastContainer.tsx

@@ -0,0 +1,42 @@
+import { useToast } from '@/hooks/useToast';
+import { X, CheckCircle2, AlertCircle, AlertTriangle, Info } from 'lucide-react';
+
+const ICONS = {
+  success: CheckCircle2,
+  error: AlertCircle,
+  warning: AlertTriangle,
+  info: Info,
+};
+
+const COLORS = {
+  success: 'bg-green-500/10 border-green-500/30 text-green-400',
+  error: 'bg-red-500/10 border-red-500/30 text-red-400',
+  warning: 'bg-yellow-500/10 border-yellow-500/30 text-yellow-400',
+  info: 'bg-blue-500/10 border-blue-500/30 text-blue-400',
+};
+
+export function ToastContainer() {
+  const { toasts, remove } = useToast();
+
+  if (toasts.length === 0) return null;
+
+  return (
+    <div className="fixed top-4 right-4 z-[9999] flex flex-col gap-2 max-w-sm">
+      {toasts.map(t => {
+        const Icon = ICONS[t.type];
+        return (
+          <div
+            key={t.id}
+            className={`flex items-center gap-3 px-4 py-3 rounded-lg border backdrop-blur-sm animate-in slide-in-from-right fade-in duration-200 ${COLORS[t.type]}`}
+          >
+            <Icon className="w-4 h-4 shrink-0" />
+            <span className="text-sm flex-1">{t.message}</span>
+            <button onClick={() => remove(t.id)} className="shrink-0 opacity-50 hover:opacity-100">
+              <X className="w-3.5 h-3.5" />
+            </button>
+          </div>
+        );
+      })}
+    </div>
+  );
+}