perf(worker): 公平调度跨节点异步准入锁

持续突发提交会通过 pg_try_advisory_xact_lock 反复抢占全局容量锁，使 Worker 补槽器在首批之后饥饿。\n\n跨进程锁改为 PostgreSQL 公平等待；进程内锁仍保证每个 API 或 Worker 最多一条连接参与等待，30 秒 lock_timeout 会回到原有重试路径，60 秒空闲事务超时继续处理失主会话。\n\n验证：临时 PostgreSQL 18 上 64 并发锁竞争峰值连接不超过 2；Go 全量测试、go vet、gofmt 通过。

apps/api/internal/store/admission_lock.go

@@ -115,18 +115,21 @@ func admissionOperationLockKeys(input TaskAdmissionInput) []string {
 }
 
 func tryAdmissionTransactionLock(ctx context.Context, tx pgx.Tx, key string) error {
-	var locked bool
-	if err := tx.QueryRow(
+	// The process-local lock limits this wait to one PostgreSQL connection per
+	// API or Worker process. Let PostgreSQL queue those process representatives
+	// fairly: repeated pg_try_advisory_xact_lock calls let a sustained submit
+	// burst starve the Worker dispatcher that must release queue pressure.
+	// lock_timeout bounds a lost-owner wait and converts it back into the
+	// existing transaction retry path.
+	_, err := tx.Exec(
 		ctx,
-		`SELECT pg_try_advisory_xact_lock(hashtextextended($1, 0))`,
+		`SELECT pg_advisory_xact_lock(hashtextextended($1, 0))`,
 		key,
-	).Scan(&locked); err != nil {
-		return err
-	}
-	if !locked {
+	)
+	if IsPostgresLockTimeout(err) {
 		return errAdmissionLockBusy
 	}
-	return nil
+	return err
 }
 
 func retryAdmissionOperation[T any](