perf(worker): 解耦准入调度与远端执行

原因：线上 P24 同构验收确认两台香港 Worker 的全局容量为 48，但跨地域逐行准入事务每轮只能形成 8 个活跃租约，队列最老等待超过 15 分钟。\n\n影响：新增可配置的异步准入 dispatcher 角色；生产两地 API 负责准入和过期回收，Worker 仅执行 River job。当前主库同站点 API 可低延迟填满容量，主库切换后另一地 API 通过既有数据库锁安全接管；未配置环境变量时保持原 Worker 一体化行为。\n\n验证：Go 全量测试、go vet、gofmt、真实 PostgreSQL 1000 任务双进程回归、前端 lint/test/build、Kubernetes server-side dry-run、Compose、ShellCheck、cluster/manual release tests 全部通过。

apps/api/internal/config/config.go

@@ -80,6 +80,8 @@ type Config struct {
 	AsyncWorkerInstanceHardLimit                  int
 	AsyncWorkerRefreshIntervalSeconds             int
 	AsyncAdmissionMicrobatchSize                  int
+	AsyncAdmissionDispatcherEnabled               bool
+	AsyncAdmissionDispatcherConfigured            bool
 	WorkerAutoscalingEnabled                      bool
 	WorkerReplicasNingbo                          int
 	WorkerReplicasHongkong                        int
@@ -188,6 +190,10 @@ 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),
+		AsyncAdmissionDispatcherEnabled:   envValue("AI_GATEWAY_ASYNC_ADMISSION_DISPATCHER_ENABLED") == "true",
+		AsyncAdmissionDispatcherConfigured: envValue(
+			"AI_GATEWAY_ASYNC_ADMISSION_DISPATCHER_ENABLED",
+		) != "",
 		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),
@@ -463,6 +469,18 @@ func (c Config) RunsAsyncExecutionWorker() bool {
 	}
 }
 
+// RunsAsyncAdmissionDispatcher keeps the legacy all-in-one/Worker behaviour
+// unless a deployment explicitly separates admission coordination from task
+// execution. Production enables this on both API sites so the instance nearest
+// the current PostgreSQL primary wins the existing database scope locks, while
+// remote Worker replicas only execute River jobs.
+func (c Config) RunsAsyncAdmissionDispatcher() bool {
+	if c.AsyncAdmissionDispatcherConfigured {
+		return c.AsyncAdmissionDispatcherEnabled
+	}
+	return c.RunsAsyncExecutionWorker()
+}
+
 func (c Config) RunsBackgroundWorkers() bool {
 	switch c.EffectiveProcessRole() {
 	case "api", "capacity-controller":

apps/api/internal/config/config_test.go

@@ -159,12 +159,30 @@ func TestProcessRolePrecedenceAndCompatibility(t *testing.T) {
 	if cfg.RunsPublicHTTP() || !cfg.RunsAsyncExecutionWorker() || !cfg.RunsBackgroundWorkers() {
 		t.Fatalf("worker role capabilities are inconsistent: %+v", cfg)
 	}
+	if !cfg.RunsAsyncAdmissionDispatcher() {
+		t.Fatal("legacy Worker role no longer runs the admission dispatcher")
+	}
 
 	t.Setenv("AI_GATEWAY_PROCESS_ROLE", "api")
 	cfg = Load()
 	if !cfg.RunsPublicHTTP() || cfg.RunsAsyncExecutionWorker() || cfg.RunsBackgroundWorkers() {
 		t.Fatalf("api role capabilities are inconsistent: %+v", cfg)
 	}
+	if cfg.RunsAsyncAdmissionDispatcher() {
+		t.Fatal("API role unexpectedly runs the admission dispatcher without explicit configuration")
+	}
+
+	t.Setenv("AI_GATEWAY_ASYNC_ADMISSION_DISPATCHER_ENABLED", "true")
+	cfg = Load()
+	if !cfg.RunsAsyncAdmissionDispatcher() || cfg.RunsAsyncExecutionWorker() {
+		t.Fatalf("API admission-only role is inconsistent: %+v", cfg)
+	}
+	t.Setenv("AI_GATEWAY_ASYNC_ADMISSION_DISPATCHER_ENABLED", "false")
+	t.Setenv("AI_GATEWAY_PROCESS_ROLE", "worker")
+	cfg = Load()
+	if cfg.RunsAsyncAdmissionDispatcher() || !cfg.RunsAsyncExecutionWorker() {
+		t.Fatalf("Worker execution-only role is inconsistent: %+v", cfg)
+	}
 
 	t.Setenv("AI_GATEWAY_PROCESS_ROLE", "capacity-controller")
 	cfg = Load()