task-29: 退避配置项（MaxMs/Factor）

task-progress-config.ts 新增 getTaskPollBackoffMaxMs()（默认 10000，
由 5000 提升）与 getTaskPollBackoffFactor()（默认 2，<1 钳制为 1）；
configureTaskPolling 支持 backoffFactor；getTaskPollBackoffMs 改用
base × factor^attempt 计算。

既有 task-086 边界测试中封顶 5000 的快照随新默认更新为 10000。
新增 8 个测试：max/factor 默认、可覆盖、非正钳制、factor 下限、
配置隔离、默认值快照。

frontend-vue/src/shared/task-progress-config.ts

@@ -18,7 +18,8 @@ export const TASK_POLL_HIDDEN_INTERVAL_MS = 60000;
 export const TASK_PROGRESS_VISIBLE_CACHE_MILLIS = 5000;
 export const TASK_PROGRESS_HIDDEN_CACHE_MILLIS = 30000;
 export const TASK_POLL_BACKOFF_BASE_MS = 500;
-export const TASK_POLL_BACKOFF_MAX_MS = 5000;
+export const TASK_POLL_BACKOFF_MAX_MS = 10000;
+export const TASK_POLL_BACKOFF_FACTOR = 2;
 export const TASK_PROGRESS_TIMEOUT_MS = 10000;
 
 export interface TaskPollingConfig {
@@ -26,6 +27,8 @@ export interface TaskPollingConfig {
   backoffBaseMs?: number;
   /** 退避上限（毫秒），必须为正数 */
   backoffMaxMs?: number;
+  /** 退避指数因子，<1 时钳制为 1 */
+  backoffFactor?: number;
   /** 页面切回前台时是否立即拉取一轮，默认 true */
   foregroundRefreshEnabled?: boolean;
   /** 前台恢复延迟（毫秒），默认 0 */
@@ -37,6 +40,7 @@ export interface TaskPollingConfig {
 const DEFAULTS: Required<TaskPollingConfig> = {
   backoffBaseMs: TASK_POLL_BACKOFF_BASE_MS,
   backoffMaxMs: TASK_POLL_BACKOFF_MAX_MS,
+  backoffFactor: TASK_POLL_BACKOFF_FACTOR,
   foregroundRefreshEnabled: true,
   foregroundRefreshDelayMs: 0,
   progressTimeoutMs: TASK_PROGRESS_TIMEOUT_MS,
@@ -62,11 +66,21 @@ export function getTaskProgressCacheTtlMs() {
   return TASK_PROGRESS_VISIBLE_CACHE_MILLIS;
 }
 
-/** 第 attempt 次退避延迟：base × 2^attempt，封顶 max；attempt 非法时按首次（0）处理 */
+/** 第 attempt 次退避延迟：base × factor^attempt，封顶 max；attempt 非法时按首次（0）处理 */
 export function getTaskPollBackoffMs(attempt: number) {
   const safe = Number.isFinite(attempt) && attempt > 0 ? Math.floor(attempt) : 0;
-  const doubled = config.backoffBaseMs * Math.pow(2, safe);
+  const factor = getTaskPollBackoffFactor();
-  return doubled > config.backoffMaxMs ? config.backoffMaxMs : doubled;
+  const scaled = config.backoffBaseMs * Math.pow(factor, safe);
+  return scaled > config.backoffMaxMs ? config.backoffMaxMs : scaled;
+}
+
+export function getTaskPollBackoffMaxMs() {
+  return config.backoffMaxMs;
+}
+
+/** 退避指数因子，<1 时钳制为 1 */
+export function getTaskPollBackoffFactor() {
+  return config.backoffFactor < 1 ? 1 : config.backoffFactor;
 }
 
 export function getTaskForegroundRefreshEnabled() {

frontend-vue/tests/task-progress-backoff-config.test.ts

@@ -0,0 +1,72 @@
+import { test } from 'node:test'
+import assert from 'node:assert/strict'
+import {
+  TASK_POLL_BACKOFF_MAX_MS,
+  TASK_POLL_BACKOFF_FACTOR,
+  getTaskPollBackoffMaxMs,
+  getTaskPollBackoffFactor,
+  configureTaskPolling,
+  resetTaskPollingConfig,
+} from '../src/shared/task-progress-config.ts'
+
+test('test_backoff_max_default', () => {
+  resetTaskPollingConfig()
+  assert.equal(getTaskPollBackoffMaxMs(), 10000)
+  assert.equal(getTaskPollBackoffMaxMs(), TASK_POLL_BACKOFF_MAX_MS)
+})
+
+test('test_backoff_factor_default', () => {
+  resetTaskPollingConfig()
+  assert.equal(getTaskPollBackoffFactor(), 2)
+  assert.equal(getTaskPollBackoffFactor(), TASK_POLL_BACKOFF_FACTOR)
+})
+
+test('test_backoff_max_override', () => {
+  resetTaskPollingConfig()
+  configureTaskPolling({ backoffMaxMs: 8000 })
+  assert.equal(getTaskPollBackoffMaxMs(), 8000)
+  resetTaskPollingConfig()
+  assert.equal(getTaskPollBackoffMaxMs(), 10000, 'reset 后恢复默认')
+})
+
+test('test_backoff_max_positive', () => {
+  resetTaskPollingConfig()
+  assert.throws(() => configureTaskPolling({ backoffMaxMs: 0 }), /退避上限必须为正数/)
+  assert.throws(() => configureTaskPolling({ backoffMaxMs: -100 }), /退避上限必须为正数/)
+  // 拒绝后 getter 仍返回正值，非正配置不可观察
+  assert.ok(getTaskPollBackoffMaxMs() > 0)
+  assert.equal(getTaskPollBackoffMaxMs(), TASK_POLL_BACKOFF_MAX_MS)
+})
+
+test('test_backoff_factor_min_one', () => {
+  resetTaskPollingConfig()
+  // factor <1 钳制为 1（不抛错）
+  configureTaskPolling({ backoffFactor: 0.5 })
+  assert.equal(getTaskPollBackoffFactor(), 1)
+  configureTaskPolling({ backoffFactor: 0 })
+  assert.equal(getTaskPollBackoffFactor(), 1)
+  resetTaskPollingConfig()
+  assert.equal(getTaskPollBackoffFactor(), 2)
+})
+
+test('test_backoff_config_isolated', () => {
+  resetTaskPollingConfig()
+  configureTaskPolling({ backoffMaxMs: 7000, backoffFactor: 3 })
+  assert.equal(getTaskPollBackoffMaxMs(), 7000)
+  assert.equal(getTaskPollBackoffFactor(), 3)
+  resetTaskPollingConfig()
+  assert.equal(getTaskPollBackoffMaxMs(), 10000, '不串到其他用例')
+  assert.equal(getTaskPollBackoffFactor(), 2)
+})
+
+test('test_backoff_max_freeze', () => {
+  resetTaskPollingConfig()
+  assert.equal(TASK_POLL_BACKOFF_MAX_MS, 10000, '默认值快照')
+  assert.equal(getTaskPollBackoffMaxMs(), TASK_POLL_BACKOFF_MAX_MS)
+})
+
+test('test_backoff_factor_freeze', () => {
+  resetTaskPollingConfig()
+  assert.equal(TASK_POLL_BACKOFF_FACTOR, 2, 'factor 快照')
+  assert.equal(getTaskPollBackoffFactor(), TASK_POLL_BACKOFF_FACTOR)
+})

frontend-vue/tests/task-progress-config.test.ts

@@ -105,9 +105,10 @@ test('test_task_086_polling_boundary_limit_and_overflow', () => {
   assert.equal(getTaskPollBackoffMs(1), 1000)
   assert.equal(getTaskPollBackoffMs(2), 2000)
   assert.equal(getTaskPollBackoffMs(3), 4000)
-  assert.equal(getTaskPollBackoffMs(4), 5000)
+  assert.equal(getTaskPollBackoffMs(4), 8000)
-  assert.equal(getTaskPollBackoffMs(10), 5000)
+  assert.equal(getTaskPollBackoffMs(5), 10000)
-  assert.equal(getTaskPollBackoffMs(100), 5000)
+  assert.equal(getTaskPollBackoffMs(10), 10000)
+  assert.equal(getTaskPollBackoffMs(100), 10000)
   // attempt 为 0 或负数按首次退避处理
   assert.equal(getTaskPollBackoffMs(-1), 500)
   assert.equal(getTaskPollBackoffMs(NaN), 500)