渠道: 模型名称映射(网关生效 + 渠道侧管理)

- 候选渠道携带 upstream_model, prepareUpstream 改写请求体 model 字段为上游名
- 新增渠道视角绑定 CRUD: GET/POST/PATCH/DELETE /admin/channels/:id/models
- 前端渠道页新增"模型映射": 列出绑定、内联改上游名、解除、添加
- rewriteModel 三种协议通用(model 均在顶层)

Co-Authored-By: Claude <noreply@anthropic.com>

server/internal/api/router.go

@@ -94,6 +94,10 @@ func NewRouter(a *app.App, gw *proxy.Gateway) *gin.Engine {
 			admin.DELETE("/channels/:id", h.AdminDeleteChannel)
 			admin.POST("/channels/:id/test", h.AdminTestChannel)
 			admin.POST("/channels/:id/models/import", h.AdminImportChannelModels)
+			admin.GET("/channels/:id/models", h.AdminChannelModels)
+			admin.POST("/channels/:id/models", h.AdminChannelAddModel)
+			admin.PATCH("/channels/:id/models/:bid", h.AdminChannelUpdateModel)
+			admin.DELETE("/channels/:id/models/:bid", h.AdminChannelDeleteModel)
 			// 模型与定价
 			admin.GET("/models", h.AdminModels)
 			admin.POST("/models", h.AdminCreateModel)