feat(billing): 完成异步结算与请求执行闭环

统一任务成功、Attempt 与结算 Outbox 的事务边界，增加多实例安全结算、人工复核、请求幂等与执行租约。钱包决策使用九位精确金额，并通过审计保护约束保留流水事实；同时补充管理接口、指标与 PostgreSQL 集成测试。

apps/api/internal/runner/execution_lease.go

@@ -0,0 +1,28 @@
+package runner
+
+import (
+	"context"
+	"time"
+)
+
+const (
+	taskExecutionLeaseTTL      = 5 * time.Minute
+	taskExecutionRenewInterval = 30 * time.Second
+)
+
+func (s *Service) renewTaskExecutionLease(ctx context.Context, cancel context.CancelFunc, taskID string, executionToken string) {
+	ticker := time.NewTicker(taskExecutionRenewInterval)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			if err := s.store.RenewTaskExecutionLease(ctx, taskID, executionToken, taskExecutionLeaseTTL); err != nil {
+				s.logger.Warn("task execution lease lost", "taskID", taskID, "error_category", "task_execution_lease_lost")
+				cancel()
+				return
+			}
+		}
+	}
+}