feat(channel): 渠道管理全链路集成 — 模型映射、定价、限制、用量统计

- 渠道模型映射：支持精确匹配和通配符映射，按平台隔离
- 渠道模型定价：支持 token/按次/图片三种计费模式，区间分层定价
- 模型限制：渠道可限制仅允许定价列表中的模型
- 计费模型来源：支持 requested/upstream 两种计费模型选择
- 用量统计：usage_logs 新增 channel_id/model_mapping_chain/billing_tier/billing_mode 字段
- Dashboard 支持 model_source 维度（requested/upstream/mapping）查看模型统计
- 全部 gateway handler 统一接入 ResolveChannelMappingAndRestrict
- 修复测试：同步 SoraGenerationRepository 接口、SQL INSERT 参数、scan 字段

backend/internal/handler/gateway_handler_responses.go

@@ -272,10 +272,7 @@ func (h *GatewayHandler) Responses(c *gin.Context) {
 				IPAddress:          clientIP,
 				RequestPayloadHash: requestPayloadHash,
 				APIKeyService:      h.apiKeyService,
-				ChannelID:          channelMapping.ChannelID,
-				OriginalModel:      reqModel,
-				BillingModelSource: channelMapping.BillingModelSource,
-				ModelMappingChain:  channelMapping.BuildModelMappingChain(reqModel, result.UpstreamModel),
+				ChannelUsageFields: channelMapping.ToUsageFields(reqModel, result.UpstreamModel),
 			}); err != nil {
 				reqLog.Error("gateway.responses.record_usage_failed",
 					zap.Int64("account_id", account.ID),