清理冗余的梯度返佣(TierCommission)配置

- 移除 Model 层：删除 ShopSeriesCommissionTier 模型及相关字段
- 更新 DTO：删除 TierCommissionConfig、TierEntry 类型及相关请求/响应字段
- 删除 Store 层：移除 ShopSeriesCommissionTierStore 及相关查询逻辑
- 简化 Service 层：删除梯度返佣处理逻辑，统计查询移除 tier_bonus 字段
- 数据库迁移：创建 000034_remove_tier_commission 移除相关表和字段
- 更新测试：移除梯度返佣相关测试用例，更新集成测试
- OpenAPI 文档：删除梯度返佣相关 schema 和枚举值
- 归档变更：归档 remove-tier-commission-redundancy 到 archive/2026-01-30-
- 同步规范：更新 4 个主 specs，标记废弃功能并添加迁移指引

原因：梯度返佣功能与一次性梯度佣金功能重复，且从未实现实际计算逻辑
迁移：使用一次性佣金的梯度模式 (OneTimeCommissionConfig.type = "tiered") 替代

internal/service/my_commission/service.go

@@ -387,25 +387,21 @@ func (s *Service) GetStats(ctx context.Context, req *dto.CommissionStatsRequest)
 		return &dto.CommissionStatsResponse{}, nil
 	}
 
-	var costDiffPercent, oneTimePercent, tierBonusPercent int64
+	var costDiffPercent, oneTimePercent int64
 	if stats.TotalAmount > 0 {
 		costDiffPercent = stats.CostDiffAmount * 1000 / stats.TotalAmount
 		oneTimePercent = stats.OneTimeAmount * 1000 / stats.TotalAmount
-		tierBonusPercent = stats.TierBonusAmount * 1000 / stats.TotalAmount
 	}
 
 	return &dto.CommissionStatsResponse{
-		TotalAmount:      stats.TotalAmount,
-		CostDiffAmount:   stats.CostDiffAmount,
-		OneTimeAmount:    stats.OneTimeAmount,
-		TierBonusAmount:  stats.TierBonusAmount,
-		CostDiffPercent:  costDiffPercent,
-		OneTimePercent:   oneTimePercent,
-		TierBonusPercent: tierBonusPercent,
-		TotalCount:       stats.TotalCount,
-		CostDiffCount:    stats.CostDiffCount,
-		OneTimeCount:     stats.OneTimeCount,
-		TierBonusCount:   stats.TierBonusCount,
+		TotalAmount:     stats.TotalAmount,
+		CostDiffAmount:  stats.CostDiffAmount,
+		OneTimeAmount:   stats.OneTimeAmount,
+		CostDiffPercent: costDiffPercent,
+		OneTimePercent:  oneTimePercent,
+		TotalCount:      stats.TotalCount,
+		CostDiffCount:   stats.CostDiffCount,
+		OneTimeCount:    stats.OneTimeCount,
 	}, nil
 }