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

- 缓存按 (groupID, platform, model) 三维 key 扁平化，避免跨平台同名模型冲突
- buildCache 批量查询 group platform，按平台过滤展开定价和映射
- model_mapping 改为嵌套格式 {platform: {src: dst}}
- channel_model_pricing 新增 platform 列
- 前端按平台维度重构：每个平台独立配置分组/映射/定价
- 迁移 086: platform 列 + model_mapping 嵌套格式迁移

frontend/src/api/admin/channels.ts

@@ -22,6 +22,7 @@ export interface PricingInterval {
 
 export interface ChannelModelPricing {
   id?: number
+  platform: string
   models: string[]
   billing_mode: BillingMode
   input_price: number | null
@@ -42,7 +43,7 @@ export interface Channel {
   restrict_models: boolean
   group_ids: number[]
   model_pricing: ChannelModelPricing[]
-  model_mapping: Record<string, string>
+  model_mapping: Record<string, Record<string, string>> // platform → {src→dst}
   created_at: string
   updated_at: string
 }
@@ -52,7 +53,7 @@ export interface CreateChannelRequest {
   description?: string
   group_ids?: number[]
   model_pricing?: ChannelModelPricing[]
-  model_mapping?: Record<string, string>
+  model_mapping?: Record<string, Record<string, string>>
   billing_model_source?: string
   restrict_models?: boolean
 }
@@ -63,7 +64,7 @@ export interface UpdateChannelRequest {
   status?: string
   group_ids?: number[]
   model_pricing?: ChannelModelPricing[]
-  model_mapping?: Record<string, string>
+  model_mapping?: Record<string, Record<string, string>>
   billing_model_source?: string
   restrict_models?: boolean
 }