渠道: Base URL 智能识别前缀/完整端点 + API 格式垂直堆叠

- upstreamURL 按内容智能拼接: 完整端点直接用 / 含 /v1 只拼资源路径 / 纯域名拼全路径
- resolveBaseURL 不再去 /v1, 交网关识别
- 渠道表 API 格式改垂直堆叠, 显示短标识 chat/completions / responses / messages
- 前端 Base URL 提示支持前缀或完整端点

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

server/internal/proxy/passthrough.go

@@ -15,6 +15,7 @@ import (
 	"time"
 
 	"github.com/gin-gonic/gin"
+	"github.com/openteam/server/internal/channel"
 	"github.com/openteam/server/internal/store"
 )