fix(security): 补齐审计日志脱敏缺口 + step-up 下载/取消体验修复

审计发现修复：

高危——审计日志请求体脱敏不全（audit_logs 沦为明文凭证聚合点）：
- 键名归一化比对（小写+去分隔符），覆盖 privateKey/apiv3key 等无分隔符与 camelCase 写法
- 程序化并入 SensitiveCredentialKeys 与 providerSensitiveConfigFields 两份权威敏感表，防清单漂移
- 补齐 proxy_key（内嵌代理密码）、custom_key（自设 API Key 明文）精确键
- Codex session 导入路由 body 整体由粘贴的 auth JSON 构成，键级脱敏无法覆盖，整体不入库
- 新增守卫测试：两份权威表的每个键必须被审计脱敏命中；provider_key 等渠道标识保留以便追责

中危——step-up 前端体验：
- 备份下载改同页 anchor 导航（预签名 URL 后端强制 attachment disposition），
  避免 step-up 弹窗 await 耗尽瞬态激活后 window.open 被浏览器拦截
- useStepUp.run 用户取消时抛 StepUpCancelledError sentinel，
  三个调用点静默处理，不再把取消误报为红色错误 toast

低危：
- 修正审计中间件挂载位置的陈旧注释（实际挂在认证之后）
- 审计 body 捕获改 LimitReader 按 256KB 上限截断读取，超出部分拼接回填，
  避免大体积导入请求被完整复制进内存两次
- TOTP step-up 弹窗验证成功后即时清空验证码输入

frontend/src/composables/__tests__/useStepUp.spec.ts

@@ -1,5 +1,12 @@
 import { describe, it, expect, vi } from 'vitest'
-import { useStepUp, isStepUpRequired, isStepUpBlocked, stepUpBlockReason } from '../useStepUp'
+import {
+  useStepUp,
+  isStepUpRequired,
+  isStepUpBlocked,
+  isStepUpCancelled,
+  stepUpBlockReason,
+  StepUpCancelledError
+} from '../useStepUp'
 
 describe('useStepUp error classification', () => {
   it('detects STEP_UP_REQUIRED from code field', () => {
@@ -60,12 +67,21 @@ describe('useStepUp.run', () => {
     expect(stepUp.visible.value).toBe(false)
   })
 
-  it('re-throws the original error if the user cancels the prompt', async () => {
+  it('throws a cancellation sentinel (not the original error) if the user cancels the prompt', async () => {
     const stepUp = useStepUp()
     const err = { status: 403, code: 'STEP_UP_REQUIRED' }
     const promise = stepUp.run(async () => { throw err })
     await vi.waitFor(() => expect(stepUp.visible.value).toBe(true))
     stepUp.onCancel()
-    await expect(promise).rejects.toBe(err)
+    await expect(promise).rejects.toBeInstanceOf(StepUpCancelledError)
+    expect(stepUp.visible.value).toBe(false)
+  })
+
+  it('classifies the cancellation sentinel distinctly from step-up errors', () => {
+    const cancelled = new StepUpCancelledError()
+    expect(isStepUpCancelled(cancelled)).toBe(true)
+    expect(isStepUpRequired(cancelled)).toBe(false)
+    expect(isStepUpBlocked(cancelled)).toBe(false)
+    expect(isStepUpCancelled({ status: 403, code: 'STEP_UP_REQUIRED' })).toBe(false)
   })
 })

frontend/src/composables/useStepUp.ts

@@ -19,6 +19,22 @@ const STEP_UP_REQUIRED = 'STEP_UP_REQUIRED'
 const STEP_UP_TOTP_NOT_ENABLED = 'STEP_UP_TOTP_NOT_ENABLED'
 const STEP_UP_ADMIN_API_KEY_FORBIDDEN = 'STEP_UP_ADMIN_API_KEY_FORBIDDEN'
 
+/**
+ * Thrown by run() when the user dismisses the TOTP dialog.
+ * Callers should treat it as a silent no-op, not an error to toast.
+ */
+export class StepUpCancelledError extends Error {
+  readonly code = 'STEP_UP_CANCELLED'
+  constructor() {
+    super('step-up verification cancelled by user')
+    this.name = 'StepUpCancelledError'
+  }
+}
+
+export function isStepUpCancelled(err: unknown): boolean {
+  return err instanceof StepUpCancelledError
+}
+
 interface ApiError {
   status?: number
   code?: string | number
@@ -76,7 +92,9 @@ export function useStepUp() {
   /**
    * Run a sensitive action. On STEP_UP_REQUIRED, prompt for a TOTP code and
    * retry once. STEP_UP_TOTP_NOT_ENABLED / admin-api-key errors are surfaced
-   * to the caller (they cannot be resolved by entering a code).
+   * to the caller (they cannot be resolved by entering a code). If the user
+   * cancels the prompt, a StepUpCancelledError is thrown so callers can
+   * distinguish "user changed their mind" from real failures.
    */
   async function run<T>(action: () => Promise<T>): Promise<T> {
     try {
@@ -91,7 +109,7 @@ export function useStepUp() {
       }
       const ok = await prompt()
       if (!ok) {
-        throw err
+        throw new StepUpCancelledError()
       }
       // Retry once now that the session holds a step-up grant.
       return await action()