refactor: 抽象统一计费会话 BillingSession

将散落在多个文件中的预扣费/结算/退款逻辑抽象为统一的 BillingSession 生命周期管理：

- 新增 BillingSettler 接口 (relay/common/billing.go) 避免循环引用
- 新增 FundingSource 接口 + WalletFunding / SubscriptionFunding 实现 (service/funding_source.go)
- 新增 BillingSession 封装预扣/结算/退款原子操作 (service/billing_session.go)
- 新增 SettleBilling 统一结算辅助函数，替换各 handler 中的 quotaDelta 模式
- 重写 PreConsumeBilling 为 BillingSession 工厂入口
- controller/relay.go 退款守卫改用 BillingSession.Refund()

修复的 Bug：
- 令牌额度泄漏：PreConsumeTokenQuota 成功但 DecreaseUserQuota 失败时未回滚
- 订阅退款遗漏：FinalPreConsumedQuota=0 但 SubscriptionPreConsumed>0 时跳过退款
- 订阅多扣费：subConsume 强制为 1 但 FinalPreConsumedQuota 不同步
- 退款路径不统一：钱包/订阅退款逻辑现统一由 FundingSource.Refund 分派

service/quota.go

@@ -307,27 +307,8 @@ func PostClaudeConsumeQuota(ctx *gin.Context, relayInfo *relaycommon.RelayInfo,
 		model.UpdateChannelUsedQuota(relayInfo.ChannelId, quota)
 	}
 
-	quotaDelta := quota - relayInfo.FinalPreConsumedQuota
-
-	if quotaDelta > 0 {
-		logger.LogInfo(ctx, fmt.Sprintf("预扣费后补扣费：%s（实际消耗：%s，预扣费：%s）",
-			logger.FormatQuota(quotaDelta),
-			logger.FormatQuota(quota),
-			logger.FormatQuota(relayInfo.FinalPreConsumedQuota),
-		))
-	} else if quotaDelta < 0 {
-		logger.LogInfo(ctx, fmt.Sprintf("预扣费后返还扣费：%s（实际消耗：%s，预扣费：%s）",
-			logger.FormatQuota(-quotaDelta),
-			logger.FormatQuota(quota),
-			logger.FormatQuota(relayInfo.FinalPreConsumedQuota),
-		))
-	}
-
-	if quotaDelta != 0 {
-		err := PostConsumeQuota(relayInfo, quotaDelta, relayInfo.FinalPreConsumedQuota, true)
-		if err != nil {
-			logger.LogError(ctx, "error consuming token remain quota: "+err.Error())
-		}
+	if err := SettleBilling(ctx, relayInfo, quota); err != nil {
+		logger.LogError(ctx, "error settling billing: "+err.Error())
 	}
 
 	other := GenerateClaudeOtherInfo(ctx, relayInfo, modelRatio, groupRatio, completionRatio,
@@ -432,27 +413,8 @@ func PostAudioConsumeQuota(ctx *gin.Context, relayInfo *relaycommon.RelayInfo, u
 		model.UpdateChannelUsedQuota(relayInfo.ChannelId, quota)
 	}
 
-	quotaDelta := quota - relayInfo.FinalPreConsumedQuota
-
-	if quotaDelta > 0 {
-		logger.LogInfo(ctx, fmt.Sprintf("预扣费后补扣费：%s（实际消耗：%s，预扣费：%s）",
-			logger.FormatQuota(quotaDelta),
-			logger.FormatQuota(quota),
-			logger.FormatQuota(relayInfo.FinalPreConsumedQuota),
-		))
-	} else if quotaDelta < 0 {
-		logger.LogInfo(ctx, fmt.Sprintf("预扣费后返还扣费：%s（实际消耗：%s，预扣费：%s）",
-			logger.FormatQuota(-quotaDelta),
-			logger.FormatQuota(quota),
-			logger.FormatQuota(relayInfo.FinalPreConsumedQuota),
-		))
-	}
-
-	if quotaDelta != 0 {
-		err := PostConsumeQuota(relayInfo, quotaDelta, relayInfo.FinalPreConsumedQuota, true)
-		if err != nil {
-			logger.LogError(ctx, "error consuming token remain quota: "+err.Error())
-		}
+	if err := SettleBilling(ctx, relayInfo, quota); err != nil {
+		logger.LogError(ctx, "error settling billing: "+err.Error())
 	}
 
 	logModel := relayInfo.OriginModelName