feat(channel): 缓存扁平化 + 网关映射集成 + 计费模式统一 + 模型限制

- 缓存重构为 O(1) 哈希结构 (pricingByGroupModel, mappingByGroupModel)
- 渠道模型映射接入网关流程 (Forward 前应用, a→b→c 映射链)
- 新增 billing_model_source 配置 (请求模型/最终模型计费)
- usage_logs 新增 channel_id, model_mapping_chain, billing_tier 字段
- 每种计费模式统一支持默认价格 + 区间定价
- 渠道模型限制开关 (restrict_models)
- 分组按平台分类展示 + 彩色图标
- 必填字段红色星号 + 模型映射 UI
- 去除模型通配符支持

backend/internal/handler/gateway_handler.go

@@ -158,6 +158,24 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 	reqStream := parsedReq.Stream
 	reqLog = reqLog.With(zap.String("model", reqModel), zap.Bool("stream", reqStream))
 
+	// 解析渠道级模型映射
+	var channelMapping service.ChannelMappingResult
+	if apiKey.GroupID != nil {
+		channelMapping = h.gatewayService.ResolveChannelMapping(c.Request.Context(), *apiKey.GroupID, reqModel)
+	}
+
+	// 渠道模型限制检查
+	if apiKey.GroupID != nil {
+		checkModel := reqModel
+		if channelMapping.Mapped {
+			checkModel = channelMapping.MappedModel
+		}
+		if h.gatewayService.IsModelRestricted(c.Request.Context(), *apiKey.GroupID, checkModel) {
+			h.errorResponse(c, http.StatusForbidden, "invalid_request_error", "Model not available in current channel: "+reqModel)
+			return
+		}
+	}
+
 	// 设置 max_tokens=1 + haiku 探测请求标识到 context 中
 	// 必须在 SetClaudeCodeClientContext 之前设置，因为 ClaudeCodeValidator 需要读取此标识进行绕过判断
 	if isMaxTokensOneHaikuRequest(reqModel, parsedReq.MaxTokens, reqStream) {
@@ -478,6 +496,21 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					RequestPayloadHash: requestPayloadHash,
 					ForceCacheBilling:  fs.ForceCacheBilling,
 					APIKeyService:      h.apiKeyService,
+					ChannelID:          channelMapping.ChannelID,
+					OriginalModel:      reqModel,
+					BillingModelSource: channelMapping.BillingModelSource,
+					ModelMappingChain: func() string {
+						if !channelMapping.Mapped {
+							if result.UpstreamModel != "" && result.UpstreamModel != result.Model {
+								return reqModel + "→" + result.UpstreamModel
+							}
+							return ""
+						}
+						if result.UpstreamModel != "" && result.UpstreamModel != channelMapping.MappedModel {
+							return reqModel + "→" + channelMapping.MappedModel + "→" + result.UpstreamModel
+						}
+						return reqModel + "→" + channelMapping.MappedModel
+					}(),
 				}); err != nil {
 					logger.L().With(
 						zap.String("component", "handler.gateway.messages"),
@@ -660,6 +693,13 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			parsedReq.OnUpstreamAccepted = queueRelease
 			// ===== 用户消息串行队列 END =====
 
+			// 应用渠道模型映射到请求
+			if channelMapping.Mapped {
+				parsedReq.Model = channelMapping.MappedModel
+				parsedReq.Body = h.gatewayService.ReplaceModelInBody(parsedReq.Body, channelMapping.MappedModel)
+				body = h.gatewayService.ReplaceModelInBody(body, channelMapping.MappedModel)
+			}
+
 			// 转发请求 - 根据账号平台分流
 			var result *service.ForwardResult
 			requestCtx := c.Request.Context()
@@ -810,6 +850,21 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					RequestPayloadHash: requestPayloadHash,
 					ForceCacheBilling:  fs.ForceCacheBilling,
 					APIKeyService:      h.apiKeyService,
+					ChannelID:          channelMapping.ChannelID,
+					OriginalModel:      reqModel,
+					BillingModelSource: channelMapping.BillingModelSource,
+					ModelMappingChain: func() string {
+						if !channelMapping.Mapped {
+							if result.UpstreamModel != "" && result.UpstreamModel != result.Model {
+								return reqModel + "→" + result.UpstreamModel
+							}
+							return ""
+						}
+						if result.UpstreamModel != "" && result.UpstreamModel != channelMapping.MappedModel {
+							return reqModel + "→" + channelMapping.MappedModel + "→" + result.UpstreamModel
+						}
+						return reqModel + "→" + channelMapping.MappedModel
+					}(),
 				}); err != nil {
 					logger.L().With(
 						zap.String("component", "handler.gateway.messages"),