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/pricing.go

@@ -37,19 +37,14 @@ func (s *Service) Estimate(ctx context.Context, kind string, model string, body
 		return EstimateResult{}, err
 	}
 	estimates := make([]candidateEstimate, 0, len(candidates))
-	var pricingErr error
 	for _, candidate := range candidates {
 		candidateBody := preprocessRequest(kind, cloneMap(body), candidate)
 		estimate, candidateErr := s.estimateCandidateV2(ctx, user, kind, candidateBody, candidate)
 		if candidateErr != nil {
-			pricingErr = candidateErr
-			continue
+			return EstimateResult{}, candidateErr
 		}
 		estimates = append(estimates, estimate)
 	}
-	if len(estimates) == 0 && pricingErr != nil {
-		return EstimateResult{}, pricingErr
-	}
 	return buildEstimateResult(estimates, pricingRequestFingerprint(kind, model, body))
 }