task-30: getTaskPollBackoffMs 递增序列

getTaskPollBackoffMs 签名改为 attempt 可选（省略视为 0，向后兼容），
序列 500×factor^attempt、封顶 maxMs 行为由任务 29 的 factor 配置驱动。
新增 8 个测试：attempt0/1/2 值、序列单调、封顶、省略参数默认、
负 attempt 当 0、自定义 factor。

frontend-vue/tests/task-poll-backoff-sequence.test.ts

@@ -0,0 +1,60 @@
+import { test } from 'node:test'
+import assert from 'node:assert/strict'
+import {
+  getTaskPollBackoffMs,
+  configureTaskPolling,
+  resetTaskPollingConfig,
+} from '../src/shared/task-progress-config.ts'
+
+test('test_backoff_attempt0_500', () => {
+  resetTaskPollingConfig()
+  assert.equal(getTaskPollBackoffMs(0), 500)
+})
+
+test('test_backoff_attempt1_1000', () => {
+  resetTaskPollingConfig()
+  assert.equal(getTaskPollBackoffMs(1), 1000)
+})
+
+test('test_backoff_attempt2_2000', () => {
+  resetTaskPollingConfig()
+  assert.equal(getTaskPollBackoffMs(2), 2000)
+})
+
+test('test_backoff_sequence_monotonic', () => {
+  resetTaskPollingConfig()
+  const sequence = [0, 1, 2, 3, 4, 5].map((n) => getTaskPollBackoffMs(n))
+  assert.deepEqual(sequence, [500, 1000, 2000, 4000, 8000, 10000])
+  for (let i = 1; i < sequence.length; i++) {
+    assert.ok(sequence[i] >= sequence[i - 1], '序列单调不减')
+  }
+})
+
+test('test_backoff_capped_at_max', () => {
+  resetTaskPollingConfig()
+  assert.equal(getTaskPollBackoffMs(5), 10000)
+  assert.equal(getTaskPollBackoffMs(10), 10000)
+  assert.equal(getTaskPollBackoffMs(50), 10000)
+})
+
+test('test_backoff_no_attempt_default', () => {
+  resetTaskPollingConfig()
+  const defaulted = getTaskPollBackoffMs(undefined as never)
+  assert.equal(defaulted, 500, '省略参数视为首次退避')
+})
+
+test('test_backoff_negative_attempt_zero', () => {
+  resetTaskPollingConfig()
+  assert.equal(getTaskPollBackoffMs(-1), 500)
+  assert.equal(getTaskPollBackoffMs(-100), 500)
+})
+
+test('test_backoff_custom_factor', () => {
+  resetTaskPollingConfig()
+  configureTaskPolling({ backoffFactor: 3 })
+  assert.equal(getTaskPollBackoffMs(0), 500)
+  assert.equal(getTaskPollBackoffMs(1), 1500)
+  assert.equal(getTaskPollBackoffMs(2), 4500)
+  assert.equal(getTaskPollBackoffMs(3), 10000, '超过 max 封顶')
+  resetTaskPollingConfig()
+})