feat(pool): 号池候选重构为 PoolCandidate 单候选模式与池内 key 故障转移

- 新增 PoolCandidate 子类，排序阶段作为单候选参与，执行阶段在 pool_keys 内部选择/切换 key
- FailoverEngine 新增 _execute_pool_candidate 方法，支持池内 key 级别故障转移与重试
- 提取 _execute_attempt / _attach_attempt_context / _classify_attempt_error 公共方法
- CandidateBuilder 对号池 Provider 构建单个 PoolCandidate（包含所有可用 key）
- CandidateSorter 支持 PoolCandidate 独立优先级分组（global_priority / pool_priority）
- CandidateResolver PRE_EXPAND 模式按 pool_keys 展开预创建记录，附加 pool_group_id
- TaskService._apply_pool_reorder 改为对 PoolCandidate 调用 select_pool_keys
- PoolManager 新增 select_pool_keys 方法，复用 reorder_candidates 逻辑
- 新增 global_priority 号池配置字段（前后端同步）
- 前端 Timeline 支持按 pool_group_id 分组显示多号池尝试
- 异步提交路径新增 _expand_pool_candidates_for_async_submit 展开逻辑

frontend/src/features/pool/components/PoolConfigDialog.vue

@@ -47,6 +47,23 @@
               同一对话始终路由到同一 Key
             </p>
           </div>
+          <div class="space-y-1.5">
+            <Label>
+              全局优先级
+              <span class="text-xs text-muted-foreground">(global_key)</span>
+            </Label>
+            <Input
+              :model-value="form.global_priority ?? ''"
+              type="number"
+              min="0"
+              max="999999"
+              placeholder="留空回退 provider_priority"
+              @update:model-value="(v) => form.global_priority = parseNum(v)"
+            />
+            <p class="text-xs text-muted-foreground">
+              global_key 模式下号池整体排序值（越小越优先）
+            </p>
+          </div>
         </div>
       </div>
 
@@ -320,6 +337,7 @@ const { success, error: showError } = useToast()
 const loading = ref(false)
 
 const form = ref<PoolAdvancedConfig>({
+  global_priority: null,
   sticky_session_ttl_seconds: null,
   lru_enabled: true,
   cost_window_seconds: null,
@@ -363,6 +381,7 @@ watch(() => props.modelValue, (v) => {
     form.value = { ...props.currentConfig }
   } else if (v) {
     form.value = {
+      global_priority: null,
       sticky_session_ttl_seconds: null,
       lru_enabled: true,
       cost_window_seconds: null,
@@ -411,6 +430,7 @@ async function handleSave() {
   try {
     const payload: Record<string, unknown> = {
       pool_advanced: {
+        global_priority: form.value.global_priority ?? undefined,
         sticky_session_ttl_seconds: form.value.sticky_session_ttl_seconds ?? undefined,
         lru_enabled: form.value.lru_enabled,
         cost_window_seconds: form.value.cost_window_seconds ?? undefined,