fix(worker): 收敛异步重准入到调度器

准入租约失效时 Worker 仅释放执行准备并短暂让出 River 槽位，由异步调度器统一恢复 waiting 或无有效租约的任务，避免多 Worker 同时争抢全局容量锁并放大数据库同步提交等待。\n\n调度扫描覆盖到期且可运行的 scheduled/available/retryable River 任务，但跳过正在执行或仍持有有效准入租约的任务；新增 PostgreSQL/River 状态集成回归测试。\n\n验证：Go 全量测试、go vet、runner/store race、gofmt、迁移安全检查通过；专用数据库集成用例因本机未配置 AI_GATEWAY_TEST_DATABASE_URL 明确跳过。

apps/api/internal/store/admission_queue.go

@@ -721,13 +721,26 @@ SELECT admission.task_id::text
 FROM gateway_task_admissions admission
 JOIN gateway_tasks task ON task.id = admission.task_id
 LEFT JOIN river_job job ON job.id = task.river_job_id
-WHERE admission.status = 'waiting'
-  AND admission.mode = 'async'
+WHERE admission.mode = 'async'
   AND task.status = 'queued'
+  AND task.next_run_at <= now()
+  AND (
+    admission.status = 'waiting'
+    OR (
+      admission.status = 'admitted'
+      AND NOT EXISTS (
+        SELECT 1
+        FROM gateway_concurrency_leases lease
+        WHERE lease.task_id = admission.task_id
+          AND lease.released_at IS NULL
+          AND lease.expires_at > now()
+      )
+    )
+  )
   AND (
     task.river_job_id IS NULL
     OR job.id IS NULL
-    OR job.state NOT IN ('available', 'pending', 'retryable', 'running', 'scheduled')
+    OR job.state <> 'running'
   )
 ORDER BY admission.priority ASC, admission.enqueued_at ASC, admission.task_id ASC
 LIMIT $1`, limit)