perf(worker): 以有界微批次提升准入吞吐

原因：线上 P24 同构验收中，单任务同步复制提交与全局容量锁串行化，使两个 Worker 的 48 个执行槽只能维持约 10–14 个运行任务，最老等待超过 15 分钟。

影响：新增可配置的 1–32 条准入微批次，默认 8；租约、任务准入与唯一 River job 在一个有界事务内原子提交，并按确定顺序预锁任务和容量范围，避免整窗 48 条大事务和多 Dispatcher 死锁。并容忍 rebind 已被其他 Dispatcher 完成的幂等竞态。

验证：Go 全量测试、go vet、真实 PostgreSQL 跨 Store 集成测试、ShellCheck、迁移安全检查、OpenAPI、前端 lint/test/build、Compose/Kubernetes 渲染及人工发布脚本均通过。

apps/api/internal/config/config.go

@@ -79,6 +79,7 @@ type Config struct {
 	AsyncWorkerHardLimit                          int
 	AsyncWorkerInstanceHardLimit                  int
 	AsyncWorkerRefreshIntervalSeconds             int
+	AsyncAdmissionMicrobatchSize                  int
 	WorkerAutoscalingEnabled                      bool
 	WorkerReplicasNingbo                          int
 	WorkerReplicasHongkong                        int
@@ -186,6 +187,7 @@ func Load() Config {
 		),
 		AsyncWorkerInstanceHardLimit:      envIntValidated("AI_GATEWAY_ASYNC_WORKER_INSTANCE_HARD_LIMIT", 32),
 		AsyncWorkerRefreshIntervalSeconds: envIntValidated("AI_GATEWAY_ASYNC_WORKER_REFRESH_INTERVAL_SECONDS", 5),
+		AsyncAdmissionMicrobatchSize:      envIntValidated("AI_GATEWAY_ASYNC_ADMISSION_MICROBATCH_SIZE", 8),
 		WorkerAutoscalingEnabled:          env("AI_GATEWAY_WORKER_AUTOSCALING_ENABLED", "false") == "true",
 		WorkerReplicasNingbo:              envOptionalIntValidated("AI_GATEWAY_WORKER_REPLICAS_NINGBO", 1),
 		WorkerReplicasHongkong:            envOptionalIntValidated("AI_GATEWAY_WORKER_REPLICAS_HONGKONG", 1),
@@ -291,6 +293,9 @@ func (c Config) Validate() error {
 	if c.AsyncWorkerRefreshIntervalSeconds < 1 {
 		return errors.New("AI_GATEWAY_ASYNC_WORKER_REFRESH_INTERVAL_SECONDS must be positive")
 	}
+	if c.AsyncAdmissionMicrobatchSize < 1 || c.AsyncAdmissionMicrobatchSize > 32 {
+		return errors.New("AI_GATEWAY_ASYNC_ADMISSION_MICROBATCH_SIZE must be between 1 and 32")
+	}
 	if c.WorkerReplicasNingbo < 0 || c.WorkerReplicasHongkong < 0 ||
 		c.WorkerMinReplicasNingbo < 0 || c.WorkerMinReplicasHongkong < 0 ||
 		c.WorkerMaxReplicasNingbo < c.WorkerMinReplicasNingbo ||