fix(admission): 按候选唤醒同步队列头

同步准入释放容量后，分别唤醒每个独立平台模型队列的 FIFO 队头；只有任务实际绑定用户组时才同时约束用户组队头，避免一个已饱和候选阻塞其他候选补位。\n\n调度器单次最多处理 256 个真实队头，不唤醒整个等待队列。新增 PostgreSQL 集成测试覆盖无用户组的双候选并行唤醒，以及共享用户组下仍保持组级 FIFO。\n\n验证：gofmt；Store/Runner 聚焦测试；一次性 PostgreSQL 集成测试；git diff --check。

apps/api/internal/store/admission_queue.go

@@ -713,10 +713,11 @@ LIMIT $1`, limit)
 	return taskIDs, rows.Err()
 }
 
-// ListWaitingTaskAdmissionIDs returns a bounded set of FIFO leaders across
-// platform-model and user-group queues. It is used after capacity is released
-// so API processes wake only plausible queue heads instead of every
-// synchronous waiter.
+// ListWaitingTaskAdmissionIDs returns one FIFO leader for every independent
+// platform-model queue, additionally requiring the task to lead its user-group
+// queue when one exists. It is used after capacity is released so API
+// processes can refill every candidate without waking every synchronous
+// waiter or allowing one saturated candidate to block another.
 func (s *Store) ListWaitingTaskAdmissionIDs(ctx context.Context, limit int) ([]string, error) {
 	if limit <= 0 {
 		limit = 256
@@ -724,6 +725,7 @@ func (s *Store) ListWaitingTaskAdmissionIDs(ctx context.Context, limit int) ([]s
 	rows, err := s.pool.Query(ctx, `
 WITH ranked AS (
   SELECT task_id,
+         user_group_id,
          priority,
          enqueued_at,
          row_number() OVER (
@@ -740,8 +742,8 @@ WITH ranked AS (
 )
 SELECT task_id::text
 FROM ranked
-WHERE platform_rank <= $1
-  AND group_rank <= $1
+WHERE platform_rank = 1
+  AND (user_group_id IS NULL OR group_rank = 1)
 ORDER BY priority ASC, enqueued_at ASC, task_id ASC
 LIMIT $1`, limit)
 	if err != nil {