feat(portfolio): max_pool 字段透传(model+handler+前端,解除30硬编码)

model 加 max_pool 字段(默认30,0=全市场不限);handler 用 req.max_pool 替换硬编码30;
前端加选股池上限 input + onSubmit 透传。下游 submit_portfolio→spec→argv--max-pool
链路异步化时已存在,本次只补入口。全周期验收可传 max_pool=0 对比 docker exec 旁证

frontend/src/views/backtest/PortfolioBacktest.vue

@@ -29,6 +29,7 @@ let pollTimer: ReturnType<typeof setInterval> | null = null
 const form = reactive({
   pool: 'hs300_subset',
   strategy: 'all_weather',
+  max_pool: 30,
   start: '2024-01-01',
   end: '2024-02-29',
   cash: 1_000_000,
@@ -108,6 +109,7 @@ async function onSubmit(): Promise<void> {
     const tid = await postPortfolioBacktest({
       pool: form.pool,
       strategy: form.strategy,
+      max_pool: form.max_pool,
       start_date: form.start,
       end_date: form.end,
       initial_cash: form.cash,
@@ -193,6 +195,16 @@ async function onSubmit(): Promise<void> {
           </el-select>
           <span class="muted form-hint">选择回测策略</span>
         </el-form-item>
+        <el-form-item label="选股池上限">
+          <el-input-number
+            v-model="form.max_pool"
+            :min="0"
+            :step="10"
+            :controls="false"
+            style="width: 220px"
+          />
+          <span class="muted form-hint">0=全市场不限, N=前N只(MVP验证用, 默认30)</span>
+        </el-form-item>
         <el-form-item label="开始日期">
           <el-date-picker
             v-model="form.start"